Why do I get so upset when my boyfriend goes out?

If he is engaging in behavior that is suspicious, then you have a right to not trust them. If he is going out with his friends all the time on weeknights, or going out late on weekends without you, and he is past a certain age and it is not really the standard behavior for his demographic, again, you have a right to be a little bit jealous. But if none of those things are going on, and you can't let your friend go out with his friends once in a while, or get jealous anytime he is talking to another person, that's a reflection on your insecurities. Then you have to figure out where these insecurities are coming from, why you're feeling this way and work on fixing them, otherwise the lack of trust will destroy your relationship over time.