In my case, I am a shy person because I grew up in an environment where I can only interact with no more than five kids other than my siblings. I basically don't know how to interact with people of varying attitudes and personalities.
Also, since I spend much of my time alone with books and devices to kill boredom, I nearly don't have time to catch up with being sociable. I am an introvert and being in a place with many people to talk to isn't just too comfy for me.
When I was in highschool, I am bullied by many of my classmates. Well, just the curses and unprofessional terms; no physical injuries in the mix. But then, I lose self confidence with that - afraid of talking to anyone else in the room and out because of that experience. It's not the worse but it's a bad feeling.
In my coming of age, I had a crush on a girl who is either fortunately or unfortunately on the same club as I am. I became so shy to her that I mostly looked like a jerk that made my shyness rise even more.
I've also had some illogical ego when I was in third year. Realizing my flaws after a year or two, I just can't face the people whom I've acted so wrong before. I feel like I'm er... Like they'll approach me and tell me my wrongdoings before and due to the fact that I know better now, I feel guilty and don't feel like I have the 'face' to face them now.
Well, that's it. Seems legit, though.