In this scene, Benvolio thinks there will be a fight because the weather is so hot. He says that in weather like that people always are in a bad mood and spoiling for a fight. So he tells Mercutio they shouldn't go out.
What Mercutio says in the lines you cite is that Benvolio always wants to fight. He says Benvolio will fight a man because the man's beard has too many hairs, or not enough hairs. What he's saying is that Benvolio will fight for any reason at all, or even for no reason. Because of this, he thinks it's weird that Benvolio wants to tell him how to avoid a fight.
Benvolio thinks that there will be a fight because the weather is hot.
The day is hot; the Capulets, abroad;And if we meet we shall not ’scape a brawl,For now, these hot days, is the mad blood stirring.
Thou hast quarreled with a man for coughing in the street because he hath wakened thy dog that hath lain asleep in the sun.