"Jealousy tends to be caused by one of two things," says Brandy Engler , PsyD, a clinical psychologist based out of LA. "It could be you're afraid of losing someone, which could point to some abandonment issues you need to work through. But usually, it's ego-related." Engler explains that the idea that you're so unique and so special that your partner would not even think of hanging out with or looking at another person is what drives this jealousy. "There's this assumption that if your partner loves you and is happy with you, then they'd never notice other men or women," she says. "That is, of course, unrealistic. The world is full of attractive and interesting people that you yourself have likely noticed, too. So it's natural your partner will."