Why does Lexapro make you gain weight?

No, but many users of the "SSRI" type antidepressants and other psychiatric medicines do experience some weight gain, maybe 5-40 lbs. Not everybody will have side effects with ANY medicine. Sometimes switching from one med to another in the same group solves the problem, no particular one. It's one of benefit/impact ratiOS you'll have to weigh for yourself. Some people actually LOSE weight because when less depressed they are more active and have fewer junk food cravings. You'll know in a month or two. Lexapro is closely related to Celexa, and is supposed to be a formulation which has fewer side effects.