Answer:
Because although everyone thinks that Benvolio is calm and pacifist, Mercutio knows that this is a lie and that Benvolio is a quarrelsome and heated person.
Step-by-step explanation:
Because Benvolio has a reputation for being calm and pacifist, but he has already got into many fights, where he presented a heated and stressed personality. Benvolio has already been involved in the same number of fights as Mercutio, who does not have this reputation as a pacifist and calm.
When Mercutio is accusing Benvolio of making trouble wherever he goes, he is playing Benvolio's hypocrisy on everyone, showing how Benvolio claims to be something he is not, just to keep up appearances.