In Act 2, Scene 7, Mirabell has a soliloquy after Millamant leaves him alone onstage. He uses an elaborate simile to describe what it is like to be a man in love with a woman:
A fellow that lives in a windmill has not a more whimsical dwelling than the heart of a man that is lodged in a woman. There is no point of the compass to which they cannot turn, and by which they are not turned; and by one as well as another, for motion, not method is their occupation. To know this, and yet continue to be in love, is to be made wise from the dictates of reason, and yet persevere to play the fool by the force of instinct.
Mirabell here reflects on his love for Millamant, describing himself as a "fool" following instinct instead of reason. Like a man who lives inside a windmill, a man in love has no sense of direction and will turn every which way based on the way the wind blows (or, in the case of the man in love, based on the ever-changing mood of the woman he loves). The simile helps the audience see that Mirabell is totally abandoned to his emotions. Millamant has the kind of power over him that the wind has over a windmill. Whereas she has treated their relationship and her rejection of him as a game, Mirabell is extremely sensitive to her comments and will do anything to win her over.
The fact that this speech is a soliloquy (there are no other characters around to deceive) helps the audience see that his love for Millamant is genuine and not performative. The genuine loss of his senses makes Mirabell's scheming and manipulation throughout the play a bit more forgivable (at least to Congreve and his intended audience) than other characters' scheming and manipulation. Mirabell engages in the same duplicity for which Congreve criticizes other characters. What makes him the hero and not another villain is his motivation: he is not seeking control or riches, but rather Millamant's love. In fact, he is so desperate for her love that he loses his otherwise reasonable sense of honor.