My new lab hates all men. He bares his teeth when he sees a man and gets all scary. He's not like that with women or children. Someone told me that's probably because the dog was abused as a pup, but that seems to be the catch-all excuse and I don't think it's backed by any research. Are there dogs that haven't been abused that simply don't like men? Is there any known cause for this? I've had other dogs who have behaved in a similar way (all adopted from the pound, so I didn't know their histories), but this guy is pretty extreme.