Act III begins with Mercutio and Benvolio discussing fighting."The day is hot, the Capulets abroad, And, if we meet, we shall not scape a brawl" (2-3). Mercutio accuses Benvolio of liking to fight so much that he will fight anyone for any reason. This, in fact, is a description of himself, which we find out later.
Tybalt enters looking to fight Romeo in retribution for his crashing the Capulet party the night before. "Boy, this shall not excuse the injuries /That thou hast done me; therefore turn and draw" (66-67). He is certainly blind to the illogical nature of taking such an extreme measure for something so minor.
Mercutio's own blind love for the fight comes into play when Romeo refuses to fight. He calls Romeo's attempt to avoid fighting "calm, dishonourable, vile submission!"(73). Mercutio then fights in Romeo's place and is killed. Tybalt and Mercutio, who have no real quarrel with each other, are blinded by their passion for the feud and the fight.
When Romeo realizes his friend died in his place, he vows revenge. "Away to heaven, respective lenity,/And fire-eyed fury be my conduct now!" (123-124). He then takes Tybalt's life, something only moments before he had no intention of doing.
This entire scene is one of people acting on impulse (passion) rather than reason.
I have added a link to the text of this scene.
I hope this helps!