Because he is that kind of guy. He loves fighting, just like he hates hell, all Montagues and Benvolio. Capulet has no fight with Romeo being a Montague, but Tybalt has to press the issue.
Basically they were spoiling for a fight. Tybalt was looking for Romeo in order to fight him, but Romeo didn't want to fight Tybalt. Mercutio didn't like Tybalt at all and was looking for a chance to fight him. He used the situation as a pretext to challenge Tybalt, who was not a man to turn down anyone's challenge.
Mercutio challenged Tybalt to a fight when there was no need to do so. Indeed Mercutio was spoiling for a fight with him from the start. When Tybalt asks if he can have a word with Mercutio, Mercutio says, "why not couple it with something? Why not make it a word and a blow?" which is about a provocative as you could get.
Mercutio does not "save Romeo" in the play by fighting Tybalt. Tybalt is about to walk away in contempt of the coward Romeo who will not fight him, and Mercutio gets involved, ostensibly to save Romeo's honour but basically just to get into a scrap. In the 1996 Baz Luhrman movie, the director has Tybalt beating Romeo when he will not fight, which does prompt Mercutio to join the fight. In that version Mercutio thinks that Tybalt will kill Romeo and that he needs to intervene to save his life. It's a more flattering read for Mercutio than what Shakespeare wrote.
Mercutio is the one who decides to fight Tybalt.
Tybalt wants to fight him. Tybalt always wants to fight someone, and he has fastened on Romeo after Tybalt spotted him at the party in Act I Scene 5.
Tybalt was asking for a fight with Romeo. Since he was married to Juliet and Tybalt was now his cousin, he refused to fight him. Then Mercutio asked to fight Tybalt and in a way kind of tempted him to fighting. Tybalt ended up killing Mercutio because Romeo got in the way when trying to stop the fight between the two. So now Romeo is angry that his friend Mercutio is dead and Tybalt doesn't have a scratch. So, Tybalt and Romeo fight and Romeo kills Tybalt.
At first, Romeo does not want to fight with Tybalt because he is now married to his kinswoman, Juliet. This makes Romeo and Tybalt of the same family, and Romeo does not want to fight his own.
Romeo kills Tybalt.
Tybalt wants to fight Romeo... :)
earlier in the play, romeo crashed a Capulet party. angry, Tybalt swore revenge with a sword fight. Romeo's friend mercutio ends up fighting Tybalt, and romeo attempts to intervene by stepping between them. Tybalt stabs mercutio, despite Romeos attempts to end the fight, and mercutio dies. Romeo and Tybalt then fight, and romeo kills Tybalt, and romeo is banished from Vienna. Merutio, romeo and Tybalt sword fight.
if you mean fight, then Tybalt after Tybalt had killed mercutio
because Tybalt wants to fight Romeo