Dont know why, maybe bullied, not much friends to speak with, rejected alot, or never tried much socialization events in life. Just do a sport, become stronger and learn teamwork, go out more by training and thinking while you jog to raise your confidence. Learn and do stuff others cant do. Try going out more with friends. Express yourself more through the phone, texting, friends and chat more somehow. Be nice, and people will be nice to you, making you feel better. And try different things, even if scared do very silly things so that you just beat the fear. Laugh more often. And stay healthy. Hope i helped, its natural, enjoy!