This scene demonstrates that Mercutio is both a jokester and a considerate, loyal friend.
At the beginning of Act II, Scene 4, Mercutio demonstrates that he is a good friend because he is worried about Romeo. He knows that Romeo is upset about the breakup with Rosaline, and it “torments him so that he will sure run mad” (enotes etext p. 47). When he hears about the challenge by Tybalt, he knows that Romeo cannot hold his own as a swordsman.
Alas, poor Romeo, he is already dead! stabb'd with
a white wench's black eye; shot through the ear with a
love song; the very pin of his heart cleft with the blind(15)
bow-boy's butt-shaft; and is he a man to encounter
Tybalt? (p. 48)
Despite his jesting, Mercutio is showing a serious side. He is truly worried about Romeo. Mercutio goes often on another flowery tangent, describing Tybalt as “more than the prince of cats” (p. 48) and describing his talent with the sword. Mercutio does not think Romeo can beat him.
When Romeo arrives, Mercutio describes him as looking like a “dried herring” (p. 49) and demands to know where he was the night before. They get into a duel of jests, with Mercutio describing him in feminine terms. Mercutio is surprised that he is in a good mood, and no longer “groaning for love” (p. 50). Juliet’s nurse arrives to talk to Romeo, and Mercutio immediately begins making crude jokes about her, returning to his playful self.