Why is my dog starting to show aggression?

My dog recently turned 2 years old, he is a Great Pyrenees and he loves everything! Recently when I take him and my other dog to the dog park, he has taken to picking out another male dog and tries to establish dominance. Sometimes the other dog shows him that he won't back down and could very quickly turn into a fight if we don't intervene and make our dog back down. Other times he picks on the one dog who submits to everyone (even though he's mive, he's loveable). Is this just his age or have these other dogs done something in dog body language that we don't see?