Prior to his plans to murder Banquo, Macbeth fails to prove to his wife that he is "man enough" to do what needs to be done...
In William Shakespeare's tragic play Macbeth, the protagonist (Macbeth) refuses to divulge his plans to murder Banquo to his wife.
Prior to his plans to murder Banquo, Macbeth fails to prove to his wife that he is "man enough" to do what needs to be done in order to fulfill the witches' prophecies and take the throne. Lady Macbeth's concerns are seen in Act I when she states the following:
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.
Prior to this statement, Macbeth had already confirmed her fears during an aside made in scene three of the same act:
If chance will have me king, why, chance may crown me,
Without my stir.
Macbeth has already stated that he will do nothing to take the crown. Instead, he will simply allow chance to insure that the crown become his whenever chance "chooses."
On a side note: Later, after the murder of Duncan, Macbeth's ambition begins to heavily outweigh his goodness. In order to keep the throne, Macbeth decides that both Banquo and Fleance (Banquo's son) must die (based upon another of the witches' prophecies that Banquo's sons will be kings). Therefore, in order to keep his throne, Macbeth orders the murder of Banquo and Fleance. Unfortunately for Macbeth, Fleance escapes and is sworn to revenge his father's murder:
O, treachery! Fly, good Fleance, fly, fly, fly!
Thou mayst revenge. O slave!
In order to prove to his wife that he is a man, Macbeth constructs a plan to rid himself of Banquo and his threat to Macbeth's crown. The conversation between Macbeth and his wife is as follows:
There's comfort yet; they are assailable;
Then be thou jocund; ere the bat hath flown
His cloister'd flight, ere to black Hecate's summons
The shard-borne beetle with his drowsy hums
Hath rung night's yawning peal, there shall be done
A deed of dreadful note.
What's to be done?
Be innocent of the knowledge, dearest chuck,
Till thou applaud the deed.
Therefore, one can see that Macbeth is looking for both her approval of his plan (once completed) and to prove that he is a man (no longer to be challenged by her).