An email I got today reminded me of when Deena's terrier mix Jake bit me. We were at the dog park and a boxer who I had known from a few months earlier came in. He immediately decided that Jake should be dispatched or lunch or something. The only one that got hurt in this was me. I grabbed Jake to get him out of th...e dog park and he whipped his head around and bit me. I'm sure he thought it was the boxer and reacted still trying to just get away.
There are many reasons why a dog aggresses or bites. Very few of them have to do with dominance, most are from fear, and some like this poor guy, from pain.