Why do some guys complain about their girlfriends to single women?

Pretty much every day he tells me he's not appreciated at home when he is the sole provider, or that he got her flowers and she hated them or he got her a present and she was mad he spent money on it. Or like talking about how she is always complaining about being fat when she's really skinny and is so insecure and jealous. Then he compliments me like "you're so sweet and caring" "you have a beautiful smile" "you are too cute". Why do guys act like they obviously like you, put down their girlfriend, but stay with them? Why not just leave her and be with me? Does he expect me to try and hook up with him?
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Or do guys just typically vent like this to their friends who are girls? With everyone else he talks about cars and stuff and makes it seem like his relationship is good but to me he just talks about being unhappy. I don't know what to think.
Why do some guys complain about their girlfriends to single women?
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