Rocky is a yr old intact GSD. He has been through obedience (we work this everyday) socialized since a young pup, he gets to go to town often and into stores (as many as I can find). We go to parks where he can see other dogs (on leash) and we go hiking too. Yesterday as we were walking into a pet store another dog ,a pup, was walking out. Pup wanted to play, was scrunch walking up to Rocky. Rocky reacted as if he was going to attack! Lunging ,barking/growl, hackles. They backed up and were going to let us through the atomatic sliding doors first but I had backed up turned my back to them ,facing Rocky and had him down/stay. I had him watching me. I also had told him "no ,leave it" when he was reacting. I don't know why he acted this way Just prior to this we had been to another pet store and he was rolling over for belly rubs from employees! He gets exposure from other dogs (he lives with 6 at home). My only thought on this is the sliding doors and a strange dog all at once?? Please offer advise on getting him to stop reacting to unknown dogs like he wants to tear them up! I did call our trainer but she is currently out of town on an extended trip.