Actually, they've studied this phenomenon -- dogs are terrible at judging the character of strangers, but they're AWESOME at detecting how you have judged the character of a stranger, even if you haven't realized (consciously) how you feel about them. Humans have awesome instincts but they SUCK at listening to them in way that dogs don't. So if Shep hates OP's fatass roommate, it's because OP hates her and he can tell. Her dog is shitlording by proxy!