In act 3 of Romeo and Juliet what does Mercutio think Romeo's reason is for not fighting?

Mercutio is a casualty of the dramatic irony which runs throughout Shakespeare's Romeo and Juliet. Dramatic irony occurs when the audience or reader knows something important that a character in a play does not know. In act 3, scene 1, Mercutio is unaware that Romeo has married Juliet, who happens to be the cousin of the "fiery Tybalt." When the men meet in the streets shortly after Romeo and Juliet's marriage, Romeo backs down to Tybalt, who has challenged him because of perceived insults. Romeo had crashed the Capulets' party the night before, and Tybalt was incensed by his presence. Lord Capulet succeeded in silencing his nephew during the party, but Tybalt vowed revenge. When he sees Romeo the next day he immediately wants to fight, labeling Romeo as a "villain." He too knows nothing of the recent marriage. Having just returned from Friar Laurence's cell where he married Juliet, Romeo is in a gracious and happy mood. He claims to love Tybalt, saying, "good Capulet, which name I tender As dearly as mine own be satisfied."
Mercutio, of course, is ready to fight. He wrongly believes that Romeo is backing down out of cowardice:

O calm, dishonorable, vile submission!Alla stoccato carries it away.

As Mercutio and Tybalt trade sword thrusts, Romeo attempts to break up the fight but inadvertently causes Mercutio to be mortally wounded. After he realizes his friend is dead, he blames himself:


This gentleman, the Prince’s near ally,My very friend, hath got this mortal hurtIn my behalf. My reputation stainedWith Tybalt’s slander—Tybalt, that an hourHath been my cousin! O sweet Juliet,Thy beauty hath made me effeminateAnd in my temper softened valor’s steel.



He seeks revenge and is rewarded when Tybalt inexplicably comes back to the scene. Romeo kills Tybalt, setting in motion the series of events which will lead to the double suicide of Romeo and Juliet later in the play.