My mastiff mix (we think he's mixed with Plott hound) is extremely submissive around other dogs. He's very social with people and likes to play even with dogs, but a lot of dogs seem to not like him, even if he tries to play.
He does find dogs who will play with him and who like him but at the dog parks or our family's homes with other dogs around, they become aggressive with him, growling, and even attacking him and biting the skin on his neck, almost as if to say "stop it". One dog, who even tho is known for being grumpy and dominant, would consistently attack him, even if our dog was happily playing with another dog.
At the dog park, a couple times when around a large group of dogs, they will start surrounding him and barking at him and chasing him away.
Even a happy friendly dog of our grandparents, started growling at him and never got used to him no matter how many times we brought him over.
I feel so bad for him because I know he wants to play. Even after being attacked or barked at, he still seems to want to be around them like he's trying to get them to like him.
Is this super submissive behavior? He's fine with people, it's other dogs that seem to not like him. We were contemplating getting another dog so he can play and socialize with another dog (of course we'd introduce before adopting to make sure they would get along ok), but I'm worried we'll get a dog who will attack him.
What can help him get along with other dogs, or is that just how he is? Why don't other dogs like him?