You get a LOT of meathead guys who want to be a cage fighter. That's all they want to be. Or they need to prove something to themselves or their friends. Even after 12 years of studying the sport, they still have something to prove, and that can be dangerous. You get in a mentality with BJJ where you are almost looking for a fight. You get confident and if you are balsy, want to test it's effectiveness in a streetfight. This is a problem for several reasons, but here's the #1:
It's a sport based martial art. There are no such things as "rules" in a streetfight. However, if you spend 2 hours a night conditioning your mind and body to react like a BJJ practitioner in the street, you could end up dead.
Example:
If I train BJJ:
I'm in a bar and a guy sees me talking to his sister. He comes over, says something rude and then shoves me hard. Depending on what level of BJJ I practice (and who's style), I've either already shot in for a double leg and taken him down or attempted any to isolate a choke from uptop. I may try to position him between me and any friends he may have while I'm choking him.
I train something slightly more effective:
I immediately shoot my fingers or a small object into his eyes or nose, temporarily blinding him on his own tears/blood. I drive my skull into his lower jaw, and if that hasn't rendered him unconscious I take advantage of his balance/positioning to lead him/flow with him to the floor, severely maiming him or choking him into unconsciousness by his collar on the way down while keeping aware through my peripheral vision of the next potential attacker/friend/guy that comes along. I do not commit to staying with him on the ground.
I'm at black ops/blackwater/level:
Prior to entering the new environment, I've observed the behavior and the dress of every person in the room. I probably haven't entered this place prior to gathering SOME sort of intelligence, but assuming I haven't, I begin to calculate who is a potential threat based on everything I can see and infer. I've adjusted my behavior and posture accordingly so that nobody could possibly view me as a potential threat. I haven't worn anything to stand out, and no alpha male is going to be threatened by my appearance. Before ANY possible fight has taken place, I have minimized the likelihood of it occurring to the lowest possibly probability.
If you're smart you do this without really being aware of it. It's called having a survival instict. I have a very close personal friend who used to be a training partner but disappeared after he became a green beret who taught me this. Not every military man is what you think he is.
Me <----needing less caffeine.