At the start of the play, Benedick is a confirmed bachelor; he swears he will never marry, and has no real interest in finding a partner in life. He is a soldier, a man's man, and only interested in male bonding and female bashing. This is particularly true when it comes to Beatrice, with whom he loves to exchange jokes and insults.
Toward the middle of the play, Benedick's friends try to play a trick on him to get him to fall in love with Beatrice. They let him overhear them talk about how much Beatrice loves him, and Benedick resolves then and there that he will be "horribly in love with her."
Benedick continues on this path toward a relationship with Beatrice, falling more and more in love with her despite the fact that it takes her a longer time to catch on and fall for him, too. By the end of the play, Benedick is ready to shave his beard, marry Beatrice, and even challenge his best friend to a duel to the death over Beatrice's cousin. He has made an abrupt and full change from a confirmed bachelor to a happy husband.