When after getting a girlfriend every woman other than her becomes not as hot. This makes your girlfriend look much more beautiful and sexy to you because she's your girlfriend. Much self-esteem is lost after a breakup with the girl and your friends will pester you for ever going out with her.
Tom: Man Jim thinks his girlfriend is the hottest girl on the earth!
Joe: Yeah, he's come down with a bad case of Girlfriend Syndrome.
Tom: I know the chick he's dating isn't even hot!