Why did Macbeth give in to lady Macbeth's provocations ?

Lady Macbeth questions his manhood. She laments that he is a coward for refusing to follow through on what he had promised her. She uses and exploits his insicurities about his male identity to get him to kill the king:

Was the hope drunk
Wherein you dressed yourself? Hath it slept since?
And wakes it now, to look so green and pale
At what it did so freely? From this time
Such I account thy love. Art thou afeard
To be the same in thine own act and valor
As thou art in desire? Wouldst thou have that
Which thou esteem’st the ornament of life,
And live a coward in thine own esteem,
Letting “I dare not” wait upon “I would, ”
Like the poor cat i' th' adage?