I have a 9-month-old neutered GSD who loves other dogs. He was socialized from the start---dogs and people. He loves going to dog parks and 90% of the time is very even-tempered. He is never aggressive and never fights. The other day a dog attacked him and he did not fight back. The dog was a lab--he could have torn him up. He plays great when he is matched with a similar size dog with similar energy level and rough housing. But 10% of the time, he'll do this, and I don't understand why. He will be playing with his good match, then a dog comes into the park that he immediately becomes obsessed with. He is neutered......and he doesn't hump them, he just won't leave them alone--to the point the owners get VERY nervous. He also doesn't hurt them--doesn't bite at all. But it's like severely intense herding. I have to watch for this behavior and get it when it happens. There is another piece to this--when he's not in this "crazed" state, he will come to me. When this switch flips, I can call for him and he will NOT come. If I try to catch him, he will dodge me, intent on that dog. What is going on? I appreciate any advice you can give.