Why does mercutio stand up to fight tybalt when romeo won't !?

Mercutio will not let Tybalt insult Romeo further. Mercutio fights Tybalt because he (Tybalt) has dishored his friend repeatedly. Although used for comoc relief, Mercutio has something in him that is dangerous. Mercutio is also frustrated that Romeo is unable to return his feelings. Fighting for Romeo's honor would seem a good outlet for his frustrations.