Why does my dog not like petting....?

My 2 year old male intact reg (trialing/show) cattle dog is very friendly and would never bite. He loves kids and he is so gentle with them (a little 4 year old girl who lives on our block loves running around with him on leash up and down the side walk. However, most adults he doesn't like being petted. He sniffs them, gives a tail wag and then goes and lays down. He is in PPT training (sence the incident i have been driving 3 3 1/2 hours one way) and even when i watch the GSD and other breeds there, they all "talk" to other people. He almost stays in constant work mode". He is VERY friendly with the people he knows WELL. He has a very dominate personality and refuses to listen to anyone but me. Does the dominate personality play a factor or what? I extensivly socalized my dog to EVERYTHING and he can be brought into a group of 300+ people (winter lights parades, kids soccer games etc) and not have one issue. Will this ignoring turn into biteing or growling? Did i not do enough socilizing with him?