As for personal protection, or bitework (sport or otherwise), I think some dogs will gladly work because they enjoy it. I talked to a guy not long ago whose dog is like that. He has a dog, his personal dog that he's done sport work and personal protection work with (not your style, but things outside the realm of traditional sport work) and that dog has had a couple live bites. The dog and reportedly others from that line bite because they want to. The training is just to teach them where and when to bite. I've seen a few sport dogs that got great joy out of working, for the reward sure, but also seemed to really enjoy the work. Some dogs really seem to get off on the fight/encounter with the decoy or agitator. They are not doing it because they "have to," they seem to really live for it.
If you think I am wrong, I am curious where and why. Additionally, if you meant something else feel free to chime in on that as well. I know there is a difference in "work" between going to the club every week or two and working all-day/every-day, the latter being something most in the States, especially suburbanites, are not likely to be particularly experienced with.
E-Dog - 2012 WPBTCA National Championships
The time when screech owls and Bandogges howl and spirits walk and ghosts break up their graves.
Act I, Scene IV of William Shakespeare's "King Henry VI"