Because Tybalt is a bully. Act 2; scene 3, Where Benvolio and Mercutio are discussing Romeo and his love for Rosaline, when Benvolio mentions Tybalt, Mercutio wastes no time in rubbishing him,to a bemused Benvolio who asks: "Why, what is Tybalt?" Mercutio then delivers a crushing condemnation of him, finishing with at least three Latin phrases none of them complimentary, to which a confused Benvolio asks: "The what?". Mercutio, "The pox of such antic, lisping, affecting phantasms, these new tuners of accent! 'By Jesu, a very good blade, a very tall man, a very good whore!' ----- why should we be afflicted with these strange flies, these fashionmongers, these 'pardon me's', who stand so much on the old form that they cannot sit at ease on the old bench!"