to mock the reasons for quarrels and feuds
In his speech Mercutio gives a list of reasons that Benvolio would quarrel with another: the number of hairs in his beard, the way he cracks nuts, the color of his eyes, coughing, waking a dog, wearing new clothes, etc. This is ridiculous because Benvolio is a peacemaker not a fighter. The Capulets arriving at the end of the conversation is fitting because Tybalt is looking to fight Romeo for showing up to the ball. Throughout the play, Shakespeare adds his own social commentary through his characters and their actions.