I have a 15 month old male Bernese Mountain Dog and he has always been really kind and gentle until the last month. Out of nowhere he began behaving aggressively to other dogs. He'll growl and charge at them over seemingly nothing. We weren't sure if it could be a pain related thing because he had surgery on both front legs mid April because of elbow displaysia and an incongruency (or something like that) but he doesn't seem to be in pain from the surgery anymore. We're getting nervous because the other day he got aggressive to our neighbors golden retriever who he has been best friends with since he we got him at 8 months when she was just coming over to say hi like she does almost daily and he's never had a problem before. Is there a cause of this and how can we solve it? Thanks!

Has he been neutered?

Yes, good question, but this is a working breed, needs a lot of stimulation, a herding instinct.

Did you know that this young dog had to have surgery; was done before you adopted him?. or you brought him through this after ? I know, lot of questions, but you leave me questing a bit :)