In romeo and juliet act 3 scene 1 why does mercutio fight tybalt?

Because tibalt doesnt liked romeo they and their families were opponents. when tibalt heard that romeo loved juliet he decided to kill him.

Tybalt is Juliet's cousin, originally, Tybalt wanted to duel Romeo instead. But since he and Juliet are married, he cannot fight Tybalt as he is basically his cousin-in-law, thus, Romeo rejects Tybalt's challenge. Mercutio finds this quite cowardly as a result he himself challenges Tybalt to fight.