Why does Romeo fight Tybalt?

Romeo fights Tybalt because Tybalt, Juliet's cousin, has killed Romeo's close friend, Mercutio. The sequence of events leading to this duel is crucial to understanding the conflict in the play.

In Act III, Scene I, Tybalt confronts Romeo, who initially refuses to fight because he has just married Juliet and considers Tybalt family. However, Mercutio, loyal to Romeo, intervenes and fights Tybalt in Romeo's stead. During the scuffle, Mercutio is mortally wounded. Consumed by anger and grief over his friend's death, Romeo avenges Mercutio by engaging Tybalt in a duel, ultimately killing him.

This tragic turn of events accelerates the downward spiral of the play, leading to the eventual deaths of Romeo and Juliet and further escalating the feud between the Capulet and Montague families.