I am back and I have tried to minimize interactions with dogs i know he does not get along with. What is interesting, is he will play nicely, running, herding, etc., with most dogs and then something will set him off. A recent interaction with a cockapoo, a good buddy, had the cockapoo flat on his back, on the ground and jax growling at him. No blood, thank goodness, but i immediately put the leash on and said, "NO" and took him home. It is a shame because he and this dog look forward to their play dates and90% of the time, they do well until we have to leave. Then he pulls on the leash, mad and growling, then he calms down as i coax him with a treat, jogging next to him, pulling on the leash. He loves a Great Dane and plays really well with Maxie. I guess my question is when do you if it is just dogs playing and the potential for one dog to injure another (which is the last thing i ever want to happen). I know another neighborhood buddy who loves to play with Jax, had a treat and was super aggressive as Jax walked near him. I had never seen this behavior from this dog, but Jax just walked away, not engaging in a fight. I jsut wish i know the reason for this impulsive behavior when his tail is wagging and playing and then something sets him off. No idea. Also, he loves to play with a tiny little mixed breed dog and is gentle, whimpers with excitement to play and has never exhibited these actions with the little dog. We have a dog park at our summer home and i used to love taking him there but i am now afraid he will get into an altercation with some larger, more powerful, stronger dog that could turn ugly. So, we walk and walk, but havent returned to the dog park since last spring. Jax loves people and has never met a stranger. When he sees the neighborhood kids, his tail wags ferociously, smiling and a joy. They run in our yard with him,whcih is great, and chase him, through his ball, and never an aggressive episode. He loves to sit with me and snuggle (unlike the previous three shelties). He is very cuddly and loving. Open to any and all suggestions. I am looking for a trainer, but like anything else, they all have different philosophies and it is finding the right one. Also, a trainer that understands a sheltie's characteristics (herding, excitemement).