In Act I Sc.5 Lady Macbeth is seen reading Macbeth's letter in which he reports to her how the prophecy that he would become the Thane of Cawdor had come true. He also tells her that the witches had prophesied that he would become the King of Scotland in the near future and that he thought it fit to convey this to her so that she could also happily anticipate that joyous event.
However, the wicked Lady Macbeth is determined that her husband should become the King of Scotland immediately and the only way that can be achieved is by murdering Duncan. But she does not trust her husband to murder King Duncan and become the King of Scotland because she knows him to be a very kind and gentle person: "too full o' the milk of human kindness." She is certain that Macbeth is a very compassionate person who will not be bold enough to kill Duncan. Furthermore she knows her husband to be an honest and straightforward man who will not adopt any wicked or false means to achieve his ambitions. She knows him to be an ambitious man, but an ambitious man who will only achieve his ambitions by "holy" means:
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."
i think she was just trying to make him want to gain her respect.....if someone you loved said you were worthless, you would what to prove them wrong