Lady Macbeth and Macbeth do love each other; however, the mutual passion for each other depends upon their dream of a mutual ("shared") greatness, a greatness that he has promised her as Lady Macbeth reminds him of it whenever Macbeth vacillates in his actions. The renowned critic, Harold Bloom writes of Lady Macbeth, who lost a son in a previous marriage,
Her power over him, with its angry questioning of his manliness, is engendered by her evident frustration--certainly of ambition, manifestly of motherhood, possibly also of sexual fulfillment.
So, she is moved both by ambition and by womanly desire.
Glamis thou art, and Cawdor; and shalt be
What thou art promised: yet do I fear thy nature;
It is too full o' the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily; wouldst not play false,
And yet wouldst wrongly win: thou'ldst have, great Glamis,
That which cries 'Thus thou must do, if thou have it;
And that which rather thou dost fear to do
Than wishest should be undone.' Hie thee hither,
That I may pour my spirits in thine ear;
And chastise with the valour of my tongue
All that impedes thee from the golden round,
Which fate and metaphysical aid doth seem
To have thee crown'd withal. (Macbeth, Act I; Scene v)
When people consider the trait of ambition, they usually refer to what an individual will do for him/herself to achieve what they feel they deserve. A person's ambition for another's success is more of a modern phenomenon. But Lady Macbeth is the predecessor of that way of thinking.
Macbeth refers to his wife in the letter that opens the scene as his "partner in greatness," which seems to indicate that she is everything he is not. While he may be a ruthless and brave warrior on the battlefield, he has taken greater pleasure in his loyalty and his status at home. This "milk of human kindness" has weakened him off the battlefield, and she needs to strengthen him with "spirits" (strong alcohol) to do what he is capable of doing , but until now has had no inclination.
There is, of course, an element of personal ambition-- who wouldn't want to be queen?--but the achievement is mostly for her husband.