The answer to this question can be found in Act I, Scene 5 of Romeo and Juliet. It is Tybalt, Juliet's hot-blooded cousin, that recognizes Romeo's voice (his face is hidden by a mask, like the other partygoers at the Capulet family ball). When Tybalt realizes that the voice behind the mask belongs to Romeo, a Montague, he flies into a rage, and orders his servant to bring him a rapier so that he can kill him. Only Lord Capulet, who does not want to cause a scene at his own party, stops him from challenging Romeo to a duel. Tybalt, after being chastised by Capulet, reluctantly agrees to withdraw, but adds ominously that "this intrusion shall / Now seeming sweet, convert to bitt'rest gall." This is an important moment in the play, as Tybalt's rage will prove to be a force that destroys Romeo and Juliet's marriage. He challenges Romeo to a duel which Mercutio, Romeo's good friend, takes up. When Mercutio dies at Tybalt's hands, Romeo kills him, an act which sees him banished to Mantua. Thus Tybalt's hatred, which first rears its head at the Capulet family ball, contributes to the destruction of Romeo and Juliet.