Don Pedro's scheme succeeds. The scheme is successful because, as stated previously, Benedick was already in love with Beatrice; but he just did not realize it on a conscious level. Benedick believes that Don Pedro and the others are telling the truth, partly because he was hidden and did not know that the others could see him, but also partly because he wants it to be true.
And because he does believe it, he decides to change his actions toward Beatrice:
Love me! Why it must be requited. (199)
The word requited means paid back or returned. Benedick immediately decides that he will now become a lover and wooer of Beatrice.
As Don Pedro suggests, Benedick examines himself. He critically judges his past behavior and realizes that he does have a major fault:
I hear how I am censured: they say I will bear myself proudly if I perceive the love come from her. They say too that she will rather die than give any sign of affection. I did never think to marry. I must not seem proud. Happy are they that hear their detractions and can put them to mending. (199-203)
Benedick is a changed man. He realizes that his worst fault (or detraction) is pride. In fact, that is the fault of both Beatrice and Benedick. Both of them let their pride get in the way of their happiness. But now Benedick intends to mend his fault. He will try not to be so proud in the future.
Benedick realizes that his companions will now laugh at him because he has always in the past spoken quite strongly against marriage. But despite that, he knows that the change in himself is best:
A man loves the meat in his youth that he cannot endure in his age. (210-11)
Change is natural, Bendick is declaring. The food (the word meat means any kind of food) we liked to eat as a child we might hate as an adult, and the food we hated as a child we might enjoy as an adult. Food, of course, is a metaphor. People's tastes change. Their attitudes change. Their desires change. In this instance, Benedick's attitude on marriage has changed.
So, Benedick decides not to let the jokes of his companions stop him from pursuing Beatrice:
Shall quips and sentences and these paper bullets of the brain awe
a man from the career of his humour? (211-13)
The words "quips and sentences" refer to the jokes and laughter that Benedick expects to receive from Don Pedro, Claudio, and others. He knows that they will tease him and poke fun at him. But to him, these jokes and laughs will be like "paper bullets." They will be harmless. And such harmless remarks will not stop him from the pursuit of his desire (career of his humour).
The pair of eavesdropping scenes (in Act II, Scene 3 and in Act III, Scene 1) are, in a sense, the highlight (the best part) of this play. The audience enjoys watching the moods and responses of Benedick as he overhears Don Pedro and the others discuss him and Beatrice. The audience enjoys watching the change occur in Benedick and hearing him trying to rationalize or justify those changes. Both this scene and the one that follows are great comedy. And the scenes are great comedy partly because of the characterization. Both Benedick and Beatrice are proud and willful and stubborn, and seeing such a dramatic change occur in them is amusing and comical.