What does the quote, "To prick the sides of my intent, but only vaulting ambition, which o'erleaps itself and falls on th'other--" (lines 27-29), mean?
Macbeth is admitting to himself that he has no justification for committing the murder of King Duncan. He specifies the reasons he has for not killing him and then candidly acknowledges in an extended metaphor that his only reason for committing the treasonous deed is his own "vaulting ambition." The metaphor, of course, refers to horses and horsemanship. His soliloquy is interrupted by the entrance of Lady Macbeth. He intended to say, "...and falls on th' other side." This metaphor suggests an inexperienced and rather ridiculous rider who tries to vault onto a horse and vaults so vigorously that he goes right over the saddle and falls in a heap on the ground. As far as this relates to Macbeth's ambition, he is foreseeing that it will be a serious mistake to murder Duncan because he has no excuse for doing so and because his misdeed will lead to his own ruin.
When Lady Macbeth interrupts his soliloquy, he tells her the conclusion he has come to in the following words:
We will proceed no further in this business.
He hath honored me of late, and I have bought
Golden opinions from all sorts of people,
Which would be worn now in their newest gloss,
Not cast aside so soon.
He is not telling her all his reasons but only what he hopes will appel to her pragmatic nature. Notice how he starts off so decisively and ends with what would seem to be a question deserving a question mark:
Not cast aside so soon?
Once again Lady Macbeth has to use her formidable powers of persuasion to rekindle her husband's ambition and resolution. She is the stronger of the two. He doesn't want to murder the King. We can hear the strong feeling of love and reverence in his voice when he asks his wife, "Hath he asked for me?" He not only loves Duncan, but he knows that Duncan loves him.
Whether or not his wife really loves him is problematical, but she has a strong hold over him. She is not only more ambitious and more ruthless but more intelligent. Macbeth is dependent upon her. Such marital relationships are not uncommon. When two people get married, one or the other will usually take the leadership and make all the major decisions for both. This can be observed in almost every marriage. Lady Macbeth "wears the pants in the family." Shakespeare waanted to make Macbeth look like a tragic hero by pinning much of the blame for Duncan's murder on the three witches, on "fate," and on Lady Macbeth. Shakespeare wanted to create a good man who performs a wicked deed, a hero who commits a heinous crime. But he makes Macbeth look weak, indecisive, feckless, uxorious, and like an athlete who is all brawn and no brains. Whenever Macbeth has to make decisions on his own, he makes a mess of things. One of his worst mistakes was to order the murder of Macduff's wife, children, and everyone else in his castle.