I remember that post about not beating up a guy he could have. I too am a "nice guy." I can't say that being a nice guy has caused problems in my life. If anything, being nice has gotten me some respect. In one situation, about a year ago some jerk who was causeing problems with some friends of mine ran into some problems. I don't want to go into details, but I helped the guy out and people asked me why. Honestly, I'd have to say that I do so because it was the right thing to do; to be nice to people even though they don't deserve it.
In relation to MMA i'm nice too, however i'm learning there's a difference between being nice and being too nice. I can thrash many less experienced people without beating them within an inch of thier life. That's not being too nice, that's being responsible. When it comes to those of either equal skill or physical ability I dont do as well because I hold back just like i hold back against the less experienced. Now that is being too nice. I myself am just learning. Think of it this way, if you're not giving your all in a fight against someone who wants your best, then you're also cheating them. Now that's not very nice is it nice guy?
If someone want's my best it's kinda disrespectful to not give it to them.
Changing your aggressive nature? I don't know, for me it's changing my thinking, not my aggressiveness. I once shoved a guy and threatened to break his arms if he didn't stop harassing one of my girl friends. Is that being a bully? Is it being of an aggressive, confrontational nature? I dont think so. I think I maintained being a nice guy (nice to my female friend).
One other note, being nice and being competitive often conflict.