Samson is now 18 months old and is a wonderful dog. I hike with him and a group of dogs daily (some known, some new each day depending) for about 2 hours off-leash. He loves it and runs and plays well with these dogs. We also do leash walks every week. He started being aggressive towards dogs his size or larger when he is on a leash about 6 months ago, and gradually he has also started acting the same towards dogs of any size when he is on a leash, or if the other dog is on a leash and Sam is not. Of course I don't like this behavior (lunging while growling, and I have to pull him away). Now, just in the past month he has surprised us 3 times now by all of a sudden, growling at people, putting his ears down and acting like he is going to bite them (or maybe just warning them?). We don't let it go any further and quickly pull him away from the person. But these people are known to him and were just petting him nicely or crouching down with their hands out to let him smell them, to say hi. He has been very well socialized since he was a 10-week old puppy when we got him. He is a big goof ball and so sweet that when he has acted like this everyone is totally shocked. Perhaps we have 2 separate issues here?...one is the leash problem, and the other is a new aggression problem for some reason such as a stage he is going thru or maybe he is sick? He seems totally healthy and got a check up at the vet last month. This past weekend we spent the entire day at a party with about 20 people mostly new people for him and he was an angel. Just wondering if anyone has any suggestions for me. Thank you!