Probably because you aren't much fun or she doesn't feel good when she's with you. You are physically, socially, and legally the most important thing in her life. If she still doesn't want to hang with you something is wrong with you or her. If it's you, change it. You can change yourself but you can't change her. If it's her, talk to her about it. If she wants to change and you can help this is a golden opportunity.