"People feel jealous because they're afraid, insecure, threatened, or scared to lose someone they love," says Jane Greer, PhD, a New York–based relationship expert and author of How Could You Do This to Me? Learning to Trust After Betrayal. A confident, well-adjusted person in a healthy partnership should rarely have a reason to get superjealous or suspicious. In short, if your relationship is a good one, you should be able to say, "I trust him"—and mean it.