Act 2 Scene 4: How do you account for Romeo's now being in the frame of mind to exchange courtier (repartee, witty remarks) with Mercutio?
During the first act Benvolio tries to engage Romeo in some witty, jovial repartee, but Romeo is not having any of it. Instead all Romeo wants to talk about is how depressed he is that the love of his life, Rosaline, is not interested in Romeo. Not only is she not interested in Romeo, but Rosaline has supposedly taken a vow of celibacy to never be with any man, let alone Romeo. Benvolio tries all manner of tactics to help Romeo get over it, but nothing works.
Later in the play Romeo is completely out of his depressed funk and completely able to engage in the verbal banter of Benvolio and Mercutio. The reason is that he is deeply and madly in love with his new "sun," Juliet. It must be some kind of love, that's for sure, because they kiss after something like 14 lines of dialogue and are talking about getting married in the next act. Romeo is certainly capable of exchanging verbal banter because he is "high on love" and nothing can bring him down.