Macbeth is a noble and respected soldier, having defeated the king of Norway. He will be rewarded for his achievements. His early interaction with the three sisters (witches) and the apparent accuracy of their prediction, convinces him that everything they say will take place. He does stop to think that he should let it take its course naturally and feels guilty at even having these thoughts -
"Let not light see my black and deep desires" (I.iv.50-52)
but, having informed Lady Macbeth of the witches prophesy, he is shamed into committing deeds that will ensure that he will be king of Scotland.
Unchecked ambition is the driving force throughout Macbeth and causes untold harm. Once he has been alerted to the possibilities for his future, Macbeth seems unable to control his desire to achieve and goes from one murderous deed to the next.
Lady Macbeth is confident and can differentiate between feelings of guilt and actual fear. She knows how to manipulate Macbeth and how to manage his guilt to ensure that he does not lose focus after killing Duncan:
wash this filthy witness from your hand" (II.i.43-44).
Her confidence - and Macbeth's need to succeed- helps her convince Macbeth that it will all be worth it when he expresses doubt.
"a little water clears us of this deed" (II.i.64)
She realises, albeit too late, that her ambition has been her undoing and is steadily destroying her relationship with Macbeth and will indeed destroy Macbeth,himself. Such is her nurturing instinct towards him - and no-one else- even though it is a
distorted, bond of love holding the two together
that she is desperate to reconnect with him, even in her madness. Only at Lady Macbeth's death does Macbeth become weary with life which "signifies nothing."