Atlas13 gave some excellent advice. I tend to be naturally aggressive and got into martial arts/contact sports to keep out of trouble (mostly successful).
That said, I think you can definitely cultivate controlled aggression. Control is the key here. People are different. For some violence is a switch that you flip. For others it’s more of a pump that you prime. I’m more of a switch flipper myself, but both can work. The trick is that it needs to be your decision to turn it on and you need to be able to stay in the driver’s seat when it gets going.
The key is control. If someone else controls your anger, they control you. This is unacceptable. If your anger takes control you get in trouble in life and make stupid mistakes in competition. Also unacceptable. If you are no longer able to process information and make rational, strategic decisions (even if those decisions are being made very fast) you have already lost the fight. Unacceptable.
Jits is actually really good for this, I’ve found.
If you try to “Hulk Out” against someone with any game at all, you will get embarrassed 10 times out of 10 unless you grossly outclass them in every other way. Wrestling, on the other hand, seems to put more of a premium on pure aggression due to the ruleset.
As far as where the aggression comes from, I think it’s a question of knowing your “why”. What are you fighting for? In sports, for most of us, I think it comes down to pride. Personal pride, team pride, family pride. Whatever your hot button is.
Speaking for myself, I just HATE to lose, period. I especially hate to lose knowing there was something more I could have given. I will go over around or through anybody or anything to avoid feeling how I know I’ll feel if I quit with something left in the tank.
Being physically dominated by another person in the way you are when you lose in a combat sport also makes me particularly wild. The guy who is trying to inflict that on me is, in that moment, my enemy. We may high five and hug it out after, but in that moment he is the white hot focus of my calculating rage.
Know yourself. Know your “why”. Understand and own your fear. Give yourself permission to be not nice. It takes work, but it’s doable.