And I wonder what the motivation for liking to fight is?
With GSDs, I think fighting with a helper or criminal, fight drive, is a form of advanced play drive. We see young puppies wrestle, chase, and "fight" each other, and it appears they are enjoying themselves. Perhaps the desire to fight is the grown-up version of that. It is FUN for the dog, especially if he knows (or thinks) he will win and won't get hurt. Some dogs fight harder if they do get hurt, I think at that point there is an element of defense (fear) and anger to the fight, and the desire to overwhelm the opponent gets kicked up a notch. Once a dog perceives that his survival is on the line, he may get an extra dose of adrenaline. Of course, at a certain point there is the possibility that he may turn and run for his life if that is an option, because GSDs do still retain survival instincts.
With dogs that truly seem to enjoy fighting and do not care if they get hurt or killed, like a game Pit Bull, it might be seen as a compulsion, a self-rewarding behavior with none of that bothersome "survival" concern. I don't think fear plays any part in it. I don't know if many people have actually seen two game Pits matched in a ring, I have only seen videos, but there is no "threat" display. No growling, no snarling, they may bark with excitement, but there is no hackling, no posturing, no nothing. The dogs are wagging their tails, eager to engage, and going after each other like they are having the time of their lives, despite the fact that they are being horribly maimed in the process. It's disturbing and fascinating at the same time, from a behavioral point of view. What is the motivation? It's not fear, could it simply be play to them? It would be interesting if you could somehow study the brain of a fighting dog while he's in the fight. How is their brain different than that of a normal dog? What hormones are flooding the brain, what parts of the brain are lighting up?
And by the same token, if it were possible to wire up a GSD's brain while he is fighting the helper, what would we find?