It's a hot day, and the "mad blood is stirring", Benvolio says at the very start of the scene. Mercutio, it seems, is looking for a fight - actively looking for trouble, even though Benvolio has told him that the Capulets are out.
Mercutio argues that Benvolio is extremely quick to anger, and quick to quarrel:
Come, come, thou art as hot a jack in thy mood as any in Italy; and as soon moved to be moody, and as soon moody to be moved.
This theory is then backed up with several examples:
Nay, an there were two such, we should have none shortly, for one would kill the other. Thou! Why, thou wilt quarrel with a man that hath a hair more or a hair less in his beard than thou hast. Thou wilt quarrel with a man for cracking nuts, having no other reason but because thou hast hazel eyes. Thy head is as full of quarrels...
Benvolio, Mercutio says, would quarrel about the most trivial of matters. In short, Benvolio is always up for a fight.
Whether you believe this character sketch of Benvolio, who throughout the play is portrayed as a peacemaker, and whose name means good-wishing, is a different story. Personally, I'm rather inclined to think that Mercutio's claims are somewhat ironic.
At the beginning of Act III, scene 1, Benvolio is nervous about the possibility of a fight, saying to Mercutio:
I pray thee, good Mercutio, let’s retire.The day is hot; the Capulets, abroad;And if we meet we shall not 'scape a brawl.
Thy head is as full of quarrels as an egg is full of meat, and yet thy head hath been beaten as addle as an egg for quarreling. Thou hast quarreled with a man for coughing in the street because he hath wakened thy dog that hath lain asleep in the sun. Didst thou not fall out with a tailor for wearing his new doublet before Easter? With another, for tying his new shoes with old ribbon? And yet thou wilt tutor me from quarreling!
Either withdraw unto some private place,And reason coldly of your grievances,Or else depart.