Did your roommate know he was a pit mix while you guys were apartment shopping? My boyfriend loves dogs. Big, small, doesn't matter. However, due to one unfortunate experience, he gets really nervous around pitbulls and pit mixes specifically. It is possible that she isn't afraid of big dogs, but is afraid of pit bulls, and doesn't want to have to argue with you about the validity of that fear. I've worked in a lot of shelters, I love pit mixes, but some people are unfortunately uncomfortable around them.

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Regardless, I don't think you should bring your dog when your roommates are voicing their objections. You chose to move in under the impression you could not bring your dog, or at least didn't bother to verify before you signed the agreement, so now I think you have to hang in there.