Tybalt as Troublemaker

Early in the play- in the first scene - Tybalt shows up just when servants of the Montagues and Capulets have been having a little street fight that Benvolio has been trying to break up.  Tybalt jumps right into the middle of the action and tries to make more fighting rather than less fighting.

When Romeo shows up at the Capulet masked ball, Tybalt overhears him giving his romantic soliloquy (also a sonnet) which begins "O she doth teach the torches to burn bright."  He complains to his uncle, Lord Capulet, who tells him to calm down which makes Tybalt very unhappy and he begins planning future revenge.

Tybalt is a hothead and a tough guy. Unlike his character foil Mercutio, Tybalt doesn't have a sense of humor. Tybalt threatens Mercutio because it is basically what he does. He is "Prince of Cats" tough guy for the Capulets. Hating Montagues is part of his identity. Mercutio knows exactly how to exploit Tybalt and get him agitated. Tybalt tries to disturb the Capulet banquet by going after Romeo but Capulet stops him. Similarly Tybalt provokes a fight with Romeo and then subsequently Mercutio.