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As for why this happens – well, talk to two trainers and you'll probably get three opinions. My thoughts are as follows:
Individual animals have different degrees of sensitivity about touch and personal space – we all know dogs like your Denver, who will moosh up to everyone, and other dogs who, while polite and friendly, will prefer to rest near their people, rather than touching. I surmise that some dogs experience bouncy, close-in body language as unpleasant, perhaps even threatening – hence "hostility" toward Labs and, sometimes, puppies.
As I mentioned, dogs with flattened faces, such as Bulldogs, seem to be on the receiving end of a lot of negative reactions from other dogs. What is it about flattened faces? Medically speaking, of course, brachycephaly is a deformity. I strongly doubt that the doggy brain is capable of stigmatizing other dogs the way children may stigmatize their unusual-looking peers. But to me it seems likely that with their heavy jaws, their pushed-out teeth, and their flat noses, Bulldogs and other brachycephalic breeds just plain look strange. And many, many dogs get spooky about whatever strikes them as strange. (This is why early puppy socialization is so important – the more varied that early experience, the fewer the things encountered in life that may seem strange.)