I have a one and a half year old Shar Pei/Boxer/??? mix that was EXTREMELY friendly for this first year of his life. However, starting in January he became increasingly aggressive to other dogs. First it was just one GSD at the dog park that he continuously got into fights with - I chalked it up to the dog's size (this was probably the tallest dog I've ever seen that wasn't a Dane or a Mastiff, which strangely enough, Chester loves). A few weeks later, he got into a fight with a Dalmatian at the park and blood was drawn (my dog's, not the dalmation's). At this point we stopped going to the park to avoid further incidents.
Since then, he has been in a room full of dogs at obedience school with no problems, played with my mom's border collie with no problems, and played with my friend's spitz with no problems. I began to assume that it was just those two dogs at the park.
Today, though, we met what looked like a pit mix, and my dog initially greeted him very friendly, then out of the blue growled and snarled at him. A second later, his tail was wagging and he was "smiling", with the other dog play bowing and Chester wagging his tail. Then when they got close again, Chester growled and snarled at the dog again. I am extremely confused as to what's going on and how to help it, especially since it isn't very predictable when his aggression will present.
Is there any way to stop this? Or can someone at least explain what is happening?