My 8 month old puppy has grown up with another dog and used to let the other dog take sticks off him without issue but in the last 2 weeks he's totally changed his mind. It started with baring the teeth and snapping at him but he's now snarling and really going for the other dog. I've been praising my dog when the other dog has come near him when he hasn't reacted but it seems to be getting worse. It's only happening with this dog that he's known since he was 8 weeks. Please help, he doesn't show any other evidence of resource gathering or with any other dog.