Why aren't Macbeth and Lady Macbeth happy being king and queen?

After receiving the witches' seemingly favorable prophecies, Macbeth reluctantly executes his wife's plan and succumbs to his ambition by assassinating King Duncan. Immediately after murdering the king, Macbeth is visibly shaken and emotionally distraught. He is overwhelmed with guilt and regrets committing regicide. Although Macbeth attains the title King of Scotland, he recognizes that he has not solidified his legacy and fears that the witches prophecy about Banquo's will come to fruition. The thought of Banquo's descendants inheriting the throne causes Macbeth significant anxiety. Macbeth reveals his inner turmoil in a soliloquy by saying,

To be thus is nothing,
But to be safely thus. Our fears in Banquo
Stick deep, and in his royalty of nature
Reigns that which would be feared ...
Upon my head they placed a fruitless crown,
And put a barren scepter in my gripe,
Thence to be wrenched with an unlineal hand,
No son of mine succeeding. If 't be so,
For Banquo's issue have I filed my mind (3.1.46–63).

Lady Macbeth recognizes that her husband is mentally disturbed and worried about Banquo and his descendants. In act 3, scene 2, Lady Macbeth elaborates on their precarious situation by saying,

Nought's had, all's spent,
Where our desire is got without content.
'Tis safer to be that which we destroy,
Than by destruction dwell in doubtful joy (3.2.5–8).

Later in the scene, Macbeth reveals that he is not content and envies the deceased King Duncan by saying,

We have scorched the snake, not killed it.
She'll close and be herself, whilst our poor malice
Remains in danger of her former tooth ...
Better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than, on the torture of the mind, to lie
In restless ecstasy (3.2.14–24).

Macbeth once again provides insight into his tortured mind by stating,

O, full of scorpions is my mind, dear wife.
Thou know'st that Banquo, and his Fleance, lives (3.2.40–41).

As the play progresses, Macbeth goes to extreme lengths to solidify his legacy and destroy his political enemies. As Macbeth transforms into a callous, bloodthirsty tyrant, Lady Macbeth becomes overwhelmed with guilt and remorse. By act 5, scene 1, Lady Macbeth has transformed into a mentally unstable queen, who sleepwalks at night and replays their violent crime. The Doctor and Gentlewoman overhear Lady Macbeth say,

Here's the smell of the blood still. All the perfumes of Arabia will not sweeten this little hand. Oh, oh, oh (5.1.45–47).

Overall, Macbeth and his wife do not find happiness after assassinating King Duncan because they are overwhelmed with guilt and anxiety. Macbeth recognizes that he cannot cleanse his soul and becomes consumed with the thought of destroying his political enemies. Lady Macbeth cannot cope with her role in King Duncan's murder and completely loses touch with reality before committing suicide.

Why aren't Macbeth and Lady Macbeth happy being king and queen?

The two malicious murderers are not happy in their ill-acquired positions for obvious reasons: they are overwrought with guilt and remorse and, therefore, cannot sleep. Macbeth has already received a portentous warning about what was to come soon after he assassinated King Duncan. We see Macbeth pronounce the following in Act ll, scene ll:

Methought I heard a voice cry 'Sleep no more!
Macbeth does murder sleep....'

Macbeth cannot enjoy his position because he is deprived of sleep after having committed his foul deed. He later, in Act lll, scene ll, tells Lady Macbeth:

But let the frame things disjoint, both the
worlds suffer,
Ere we will eat our meal in fear and sleep
In the affliction of these terrible dreams
That shake us nightly: better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy.

It is obvious that Macbeth is suffering from insomnia. He states that his mind is tortured and that it would be better to be dead than suffer the sleepless agony brought on by his guilt. 

In addition, he becomes paranoid and goes about having all those he believes are his enemies or a threat, such as Banquo, assassinated. He has Macduff's entire family and his servants executed. He is later tormented by Banquo's ghost. In Act lll, scene lV, his horror and his guilt are apparent when he addresses what he believes is Banquo's spirit at the banquet table:

Thou canst not say I did it: never shake
Thy gory locks at me.

In Act V, scene l, we discover that Lady Macbeth has been suffering the same perturbation experienced by her husband. Her gentlewoman informs the doctor that she has witnessed her walking in her sleep.

I have seen
her rise from her bed, throw her night-gown upon
her, unlock her closet, take forth paper, fold it,
write upon't, read it, afterwards seal it, and again
return to bed; yet all this while in a most fast sleep....

She furthermore mentions that Lady Macbeth has become afraid of the dark and that she consistently rubs her hands as if she is washing them. When Lady Macbeth later cries out, we learn exactly what it is that's troubling her.

Out, damned spot! out, I say!--One: two: why,
then, 'tis time to do't.--Hell is murky!--Fie, my
lord, fie! a soldier, and afeard? What need we
fear who knows it, when none can call our power to
account?--Yet who would have thought the old man
to have had so much blood in him.

In the end she cannot live with the guilt anymore and commits suicide.

In scene lll, we discover that Macbeth has grown tired of his way of life and wishes for some relief:

I am sick at heart,
When I behold--Seyton, I say!--This push
Will cheer me ever, or disseat me now.
I have lived long enough: my way of life
Is fall'n into the sear, the yellow leaf;
And that which should accompany old age,
As honour, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honour, breath,
Which the poor heart would fain deny, and dare not.

The Macbeths have realized that their "overriding ambition" has not brought them the reward and fulfilment they thought it would. Throughout their reign they have been plagued by the results of their malice and have been living lives mostly overwhelmed by the torture and pain of their guilt.

Why aren't Macbeth and Lady Macbeth happy being king and queen?

Once Macbeth kills King Duncan, he hears the words "Sleep no more." That is exactly what happens to both of them.  Because of their guilt, they are both insomniacs.  They cannot sleep at night.  In fact, in Act V, Scene 1, Lady Macbeth is observed sleepwalking.  In that scene she reveals her secrets and the Gentlewoman and the Doctor overhear her words and feelings of guilt.

"Yet who would have thought the old man
to have had so much blood in him?"

While Lady Macbeth is slowly going crazy without sleep and the overwhelming feelings of guilt, Macbeth is becoming more and more paranoid.  He wants to ensure that no one will be in his way.  He immediately (after becoming king) hires murderers to kill Banquo, once his best friend, and also his son Fleance.  To ensure that the murder occurs, he even hires a third murderer to join them--just to make sure the job gets done.  Macbeth talks the first two men into hating Banquo so he knows that they'll go through with the killing.

"Both of you
Know Banquo was your enemy"

After Banquo, he goes after Macduff's family.  It's never ending. 

Both Macbeth and Lady Macbeth can never really enjoy their royalty because of both the lack of sleep (guilt) and because of his paranoia.

Why aren't Macbeth and Lady Macbeth happy after becoming King and Queen of Scotland?

I'd suggest that their guilt is destroying both of them as individuals and as a couple. 

Lady Macbeth, who initially persuaded Macbeth to kill King Duncan but who does not do the deed itself because Duncan looks too much like her own father, is overcome with guilt over her part in the murder.  The memory of all the blood she saw (because she had to retrieve the daggers that Macbeth left at the scene of the crime) prevent her from sleeping and later cause her to walk and talk in her sleep, reliving parts of the crime again and again.  She has lost her husband, in that he no longer talks to her about important things, much less love, and she feels no joy in being queen.

Macbeth, while initially wavering about murdering Duncan, has become quite mad and is also no longer able to sleep.  He has had to murder again (Duncan's servants) and has ordered the murders of friends and children in order to secure his throne.  These murders, however, just make him worry more about who will try to take the crown from him.  He can trust no one - and no longer confides even in his wife.  He feels alone and recognizes that there is no way to recover what he once had, and his future looks grim, even if he has the power he once wanted. 

In Macbeth, why aren't Macbeth and Lady Macbeth content as king and queen?

There are several reasons why both Macbeth and his wife are unhappy in their esteemed positions of king and queen. After murdering King Duncan to take the Scottish throne, Macbeth fears that the witches' prophecy regarding Banquo and his descendants will come true. Macbeth becomes concerned about cementing his legacy and is preoccupied with killing Banquo and his son, Fleance, to prevent their descendants from ruling as future kings. Macbeth feels that assassinating King Duncan and damning his soul will not be worth anything if Banquo's descendants will simply inherit the throne.

Macbeth also fears his political opponents and develops into a paranoid, violent tyrant. Macbeth stations spies in the homes of Scottish nobles and proceeds to carry out a violent killing spree to vanquish his political opponents. The constant murder and bloodshed negatively affect Macbeth's state of mind.

Macbeth and his wife are also unhappy as king and queen because they suffer from overwhelming guilt and fear. Macbeth experiences hallucinations and even sees Banquo's ghost shortly after he is assassinated. Lady Macbeth also shows signs of mental anquish and begins to sleepwalk. Lady Macbeth also experiences hallucinations and believes that she cannot wash King Duncan's blood from her hands. She becomes so overwhelmed with guilt and depression that she eventually commits suicide.

In Macbeth, why aren't Macbeth and Lady Macbeth content as king and queen?

Macbeth and Lady Macbeth are not happy being king and queen because they do not feel that they are safe in their positions.  Macbeth remains feeling ultimately guilty for killing first Duncan and then Banquo, and his guilt causes him to have fits and hallucinations.  He tells his wife that he has been having nightmares and that he does not have a general sense of peace.  The couple fear that someone will find out what they have done, so they cannot rest easily.  Finally, Macbeth fears that the second half of the witches' prophecy will come true and that Banquo's line will usurp him to take over the throne.  How can the couple be happy when they are in constant fear that their time in power is short-lived?

In Macbeth, why aren't Macbeth and Lady Macbeth content as king and queen?

The renowned critic Harold Bloom calls Shakespeare's Macbeth "a tragedy of the imagination," contending that Macbeth and Lady Macbeth derive satisfaction in their wickedness. However, this satisfaction does not last as their imaginations rage out of control in suspicion, turmoil, and paranoia. Macbeth in his "vaulting ambition" must continue doing evil even though he greatly suffers from knowing that he does evil. After he kills Duncan, for instance, he then fears the witches' prophecy about Banquo and his descendents and then makes his murderous attacks upon Banquo and his son.

Likewise, the apparently coldly brutal Lady Macbeth is halted in her murderous path by conscience.  Ironically, when she boldly urges Macbeth to slay Duncan and ridicules her husband for his twinges of conscience as he says "I am afraid to think what I have done" (2.2.65) by retorting, "A little water clears us of this deed" (2.2.85), like Dostoevsky's Raskolnikov, she does not consider that her own conscience will haunt her. Thus, by Act V she has gone mad from the disquiet of her guilt and kills herself. Macbeth, so consumed with raging ambition and lust for power that he is more concerned with preparing himself for battle in Dunsinane, does not even inquire how his wife has died when he is informed.  Later however, even in his moral turpitude, he feels again the twinge of conscience,

My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honor, breath,
Which the poor heart would fain deny and dare not. (5.3.25-31)

Unable to control their imaginations and consciences, Macbeth and Lady Macbeth are disquieted by the deeds that their raging ambition have wrought.

Why aren't Macbeth and Lady Macbeth happy as king and queen after reading Act 3?

In adding a few details to the previous answer, I would like to begin with the idea that very few people are satisfied with what they have. Macbeth is no different. 

First of all, power makes him forget a few key prophecies the witches tossed to Banquo, mainly that Banquo's sons will be king. Only after he kills Duncan does he realize that "to be thus, is nothing; but to be safely thus" (III, i, 51-52).

In order to be safe, he has to continue killing, beginning with his best friend and his son. Unfortunately, Fleance escapes, heightening Macbeth's fear that "For Banquo's issue have I filed my mind; / For them the gracious Duncan have I murder'd; . . ." (III, i, 68-69).

With this realization, Macbeth not only becomes unhappy and angry, but he begins his descent into insanity and evil. His embarrassment at his own coronation banquet can never be undone. Other than covering for his insane behavior, Lady Macbeth has no more impact upon Macbeth's activities. Their apparent bond has been broken. 

The final scene of this act confirms Macbeth's undoing. Instead of a kingdom full of admiring and loyal thanes, a real rift has developed among the men. This divide is recounted by Lennox and another nobleman. They note that Macduff has denied Macbeth's invitations to the castle on two occasions, thereby placing him in direct conflict with Macbeth. He has fled to England to reunite with Malcolm and hopefully find that "...a swift blessing / May soon return to this our suffering country / Under a hand accursed!" (III, vi, 8-10).

It is clear that Macbeth and his wife are not enjoying their power. She is watching her husband descend into insanity and secrecy while he loses both his friends and the support of the nation.

Why aren't Macbeth and Lady Macbeth happy as king and queen after reading Act 3?

Macbeth has begun to realize that his time as king is limited. He knows that the witches proclaimed a long line of succession to Banquo, and his failure to dispose of Fleance has him worried to the point of hallucination. Banquo's ghost makes a couple of appearances at Macbeth's banquet, and Macbeth speaks aloud to it in front of all of his guests. Lady Macbeth is left to try and come up with an explanation for Macbeth's odd actions.

In addition, the absence of Macduff at the banquet has Macbeth assured that Macduff knows something is up. Not only does Macbeth now have to be concerned about Fleance eventually starting a line of kings, but Macbeth's own noblemen are out building armies to oppose him. The only solution he can think of is to revisit the witches for a clarification to his original prophecies.