As they are wrestling to control and not to make others go away, their biting behavior is not defensive. The bites are not just a nip, but rather a grasp and hold. Once fighting dogs are given the opportunity to fight, they may look forward to the next opportunity with great anticipation. Dogs that fight are often eager to mix it up with another dog. This too, is addictive to the dog.
Many dogs are prey driven.
Prey driven dogs tend to chase things that move fast. They may be seen stalking the object of their interests. When they grasp the object, they often shake it, which would serve to kill game, as the dog's wild ancestors did. It is often assumed that the game pit fighting dogs are motivated by prey behavior. They can really get hooked on it, if given the chance.