Discuss the line "Ambition, madam, is a great man's madness."

Antonio is a humble and straight man—or at least he appears to be so among an incredibly crooked court. He speaks this line in Act I, Scene 3 to the Duchess, with whom he is having a conversation heavy with implication and innuendo. For example, the Duchess says that one of Antonio's eyes are bloodshot, and she gives him her ring for its "healing properties." This is of course her wedding ring, and she is implying that Antonio should marry her. She goes on to say that his head is too low, and she needs to "raise him up" to talk to him. She means this in terms of court status, not literally. It is then that he says "Ambition, madam, is a great man's madness." Antonio is ambivalent in regard to pursuing higher things and thinks none of this would be worth the trouble and scandal it would cause. He fears the intoxicating effects of power and how they may change him from his righteous ways.

The line "Ambition, madam, is a great man's madness" is spoken by Antonio in act I, scene iii. It shows Antonio's ambivalence at the prospect of marrying the Duchess. Antonio is merely a humble steward, the man who runs the Duchess's household. He knows that marriage between an aristocratic lady and one of her servants would cause a huge scandal in such a class-conscious society.
Furthermore, Antonio is concerned that being elevated to such a high position in life will tempt him to become ambitious, and that way madness lies. (Just one look at some of the other characters at court tells us that Antonio is absolutely right to be worried.) Antonio is a fundamentally decent man, a man of great integrity who's generally happy with his lot in life. Although marrying the Duchess will give Antonio an entree into a world of wealth, glamor, and high social status, it could also just as easily corrupt his soul, making him ambitious for riches and power. Antonio senses the dangers of getting married to the Duchess, and so he tries to talk her out of it.