Give the relationship some time. She might look or smell like someone who mistreated your dog in the past. Or her scent/perfume/smell of her own dogs is somehow offensive to the dog. Your dog might also be jealous of your affection towards her. Don't let your dog pick your girlfriend...If she seems good to you so far, then stick with her. In time, the dog will get used to her...

Kathy's right, I'd bet it's the smell ... get ur your girl to wear one of your shirts when she comes over next time, see if he responds differently.....
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And wouldn't that be just terrible lol, if every time she comes over she has to strip down and wear nothin' but the shirt off your back..
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If it works maybe you're dog just trying to be ur wing man lol ... I know Carlos loves when I parade around wearing his...
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Angie..

My dog has always been right...
It could be that your dog doesn't like the smell of her dogs. Has your dog met her dogs?..
I dated a guy who my dog would not be in the same room with. She was right. ..
I have a friend who had to put her dog on anti-depressants so it could get along with her boyfriend and her dog. She should have listened to her dog. Because that was the all-time worst relationship ever and she could have saved herself some heartache...

Let me get this straight... your dog is picking your girlfriends?..
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You do realize that dogs are territorial and it sees her (rightly) as an invader of it's territory. right?..
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My guess is that you are probably a better judge of character than your dog. Wrapping her in some of your clothes and bearing some treats and affection will ease much of the tension, that and showing your dog who's the boss...
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-Carlos..
Angie makes my howl...

I can see why you'd be concerned, but although I'm rather new to living with dogs myself, I do know that animals can just take dislike to a person and you never find out why. It could be that she is sleeping in your bed where your dog wants to be and that you are petting her instead of your dog. He'll have to just get used to her or else go outside or into another room when she comes over...