The literary critic Stephen Greenblatt has designated Mercutio as a force within Shakespeare's play that acts to "deflate" the possibility of romantic love and the power of fateful tragedy. Certainly, in Act III, Scene 1, Mercutio's actions establish the "tragedy of chance" as Ruth Nevo assesses.
In this scene, Mercutio perceives himself as the martial hero who must defend the honor of his friend Romeo. Since he has witnessed Romeo's psychological shattering over Rosaline's rejection of him and his lack of moderation in his new infatuation demonstrated by his clambering over the walls of the Capulet orchard just to stand under Juliet's balcony in the moonlight in the desperate hope of catching sight of the daughter of his enemy, Mercutio feels that he must defend the honor of the Montague family against the insults of Tybalt. He also perceives Romeo as more susceptible to harm since he is so foolishly in love with Juliet.
Earlier, too, in Act II, Scene 1, Mercutio alludes to Romeo as a "madman" for falling for someone else so quickly. At one point, too, when he and Benvolio cannot find him, Mercutio calls him a silly ape--"the ape is dead"-- that must be found. So, Mercutio probably feels that Romeo is no match for Tybalt since he is in such a state of weakening infatuation.