I think we all go through similar phases as teenagers due to hormone imbalances, but there are definitely ways to improve. As a teenager, I was very reclusive and avoided social interaction in any way possible. I would go through entire semesters of school where I would say literally no words the entire time I was in classes or in down times. If I was interested in a girl (granted there weren't very many), I'd just admire from a distance and dream about them for upwards of a year or more before life circumstances would prevent us from being in the same place. Some people were very out reaching and nice to me - even inviting me to things - and I'd find ways to weasel out of things before they eventually lost interest in trying to be my friend.
It wasn't until I got a job last year that I started getting attached to people and they started getting attached to me. In the work setting, I was expected to be exuberantly friendly to people and communicate with coworkers. Before too long, I was making small talk with strangers, making jokes with people I worked with, and I now seem to be getting into a romantic affair. If you keep forcing yourself to be friendly to people, it will get easier with time and they'll want to be around you. Rejection tends to be rare but anytime it happens to me, I just brush it off. It may linger for a short time but the past is done and dwelling in it doesn't help. Once again, that'll get easier as well.
