A girl sees a guy with a girlfriend. Girl's monkeybrain says "she thinks he's a good bet, so he's already been vetted, and he's probably a good bet."
Now a girl sees a guy at a party with two girls flirting with him. Now two other girls think he's a good bet. He must be a good bet.
Now scale up. One guy is on stage playing the guitar. 3000 women are screaming that they love him. Our girl's monkeybrain: "literally thousands of other girls are debasing themselves to be with him - he is the most desireable guy in the world. Quick, scream, get his attention! He's your perfect soul mate, just make him notice you!"
