Mercutio is under the mistaken assumption that Romeo is being calm with Tybalt in Act 3, scene 1 because he is afraid. It is said throughout the entire play that Tybalt is an expert swordsman. Mercutio and Benvolio have also been discussing the fact that they are afraid of Tybalt coming to fight the duel with Romeo because they fear Romeo is too distracted by his love for Rosaline to be of any use in a fight. When Romeo tries to placate Tybalt by telling him that he loves him more than he can understand, Mercutio takes it as a sign of weakness, of flattery and stands up for his friend. In short, Mercutio thinks fear and immobilization because of love sickness stall Romeo from fighting Tybalt in Act 3.