Mercutio is prone to expressing strong views. One key moment when he does so occurs in act 3, scene 1.
In this scene, Tybalt, a hothead who has been looking for a fight with Romeo ever since seeing him flirt with Juliet at the Capulet ball, catches up with him on the streets of Verona. Romeo has just secretly married Juliet, and the last thing he wants is a fight with his new wife's cousin. As Tybalt taunts him, trying to goad him into a sword fight, Romeo reacts calmly. He tells Tybalt that his rival doesn't know how much he loves him, that he doesn't want to fight, and that the prince has forbidden it. Romeo's reasonable, conciliatory words ignite Mercutio's fury. He thinks his friend is being a sniveling coward and says,
O calm, dishonourable, vile submission!
At this point, the die is cast. Mercutio has become as hotheaded as Tybalt and is determined to fight him in place of Romeo. Mercutio's strong reaction and views on fighting cause him to draw his sword. This is what Tybalt wants, and a fight begins—tragically, in front of the besotted Romeo, who tries to intervene, distracting Mercutio and allowing Tybalt to strike the fatal blow that kills his close friend. This incites Romeo to kill Tybalt and then end up banished from Verona. Mercutio's rash action sets into motion the tragic series of events that dominate the second half of the play.
Mercutio's words raise the tension because of the play's dramatic irony: we know as an audience what Mercutio does not, that Romeo has good reasons not to fight. Now we worry all the more about the implications of this fight.
Mercutio is often too quick to impose his own opinions. He ignores and pooh-poohs the dream in which Romeo has forebodings of early death and then rejects Romeo's desire to pursue peace with the Capulets, causing his own death.