A lot of people here are big on rescue, where using dog psychology isn't always feasible. Sorry Rick and Lida, I am only going to hijack for one quick second
When dogs come through my house, they are usually trouble children to begin with, deemed un-trainable. I usually foster/adopt the staffy bully breeds, which on their own can be very bull headed. A lot of times I (or we at the shelter) don't have time to psycho analyze the dog, we have a short period of time to prove that this dog, despite the lack or previous training or being tied to a shed for it's miserable 5 years of exsistance, can in fact be trained and excel as a loving family pet.
There are different training methods some work and some don't. No two dogs are the same, and the learning curve and train-ability will always be different. I think there is great stuff on this thread about training ideas!