The first factor causing revenge springs from the ancient grudge between the Capulets and the Montagues, which curiously was never fully explained in the play. This ancient grudge is the initial justification that accounts for the two family's first dispute in the streets of Verona.
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean. (Prologue)
However, after the first civil brawl, Romeo and Mercutio decide to intrude on the Capulet's masked party. Consequently, they eventually become discovered by Tybalt due to Romeo's easily distinguishable voice, yet Capulet discourages and scolds him from confronting Romeo and ruining the party. This humiliates Tybalt, and draws him into wanting to take the present matters into his own hands, dealing with Romeo and Mercutio some other time while his anger gradually builds.
Tybalt
This, by his voice, should be a Montague.
Fetch me my rapier, boy.
Capulet
Am I the master here, or you? go to.
You'll not endure him! God shall mend my soul!...
Well said, my hearts! You are a princox; go:
Be quiet, or--More light, more light! For shame!
I'll make you quiet. What, cheerly, my hearts! (1.5)
After the Montague's intrusion on the masked party and the first civil