That just means they are really insecure. My old boss, I was good friends with her daughter for a good year before she started dating her BF, to this day which is 3 or 4 years later..........he still hates me because he thinks I'm trying to nail his GF........he also doesn't like any of her guy friends and never trusts her.......he is just one of the most pathetic people I've ever met......out of HS, has no job and doesn't go to school and lives with his mom, i can really see the attraction huh?

Back on topic though, I would tell them to break it off because its not worth it to be with someone like that who isn't going to trust u and IMO a relationship should be about trust........so.......I would tell this person to break it off and go look for someone more mature and sure of themselves.