1) It's just too sweet: Ah, the craving, it just doesn't fade, does it? However forgiving a person you are, the mental satisfaction you acquire as soon as this dish called revenge is served cold is absolutely impeccable. No amount of guilt can overbear this sweet, satiating feeling!
2) Let us be fair: You might tell me "life isn't fair!" But justice comes to those who seek it!
3) It helps you move on: I've had fights with friends many million times, and I proudly proclaim that I'm a pugnacious moron. And if fights don't resolve things, they definitely aid you in moving on with life.
4) Self-esteem rebuilt: People have called it ego, hence I let things go, in the past! But a person without a backbone might as well be a slug! Bleh!
5) They always come around, they always do: Forgiving and forgetting gives people chances to make the same mistake repeatedly. Vengeance makes them realize it, apologize and become a better person.
All I'm saying is, never ever intentionally hurt anyone, but if they do, punch them in the face, and never look back!
"Man cries, his tears dry up and run out. So he becomes a devil, reduced to a monster."- Kohta Hirano