What do you think caused your avoidant personality?

No offense, but your dad sounds like a real asshole. I actually found myself getting angry while reading your post.

That's actually not the end of it. The worst thing he's ever said was that he thought I was going to drop out of college if I were to go. This was during my junior year of high school, and he said it behind my back to my mother, who told me secretly. You would think that maybe he said it to motivate me to do well, but if that was the case, wouldn't he say it directly to me? Oh well, I've proven him wrong so far, since I'm in my senior year now....sigh

I'll spare you of your anger by stopping here with stories about my father.

Some amount of genetic predisposition, social learning and operant conditioning.
My mother has an extreme personality (probably something between histrionic and borderline) and I'm quite sure she conditioned me for my fear of rejection. I received lots of attention and comforting when she was well of, but the next second I made the slightest mistake (looking in the wrong direction, saying sth stupid, not tidy up my room) and she would freak out completely, assault me, hit me and lock me up in my room and after this not talk to me for days. I didn't know it was her personality, I really believed it was my own mistake and that I was just socially incabable and a bad person anyways. That's the conditioning part. I tried to avoid conflicts and be invisible more or less.
The Social learning part comes from my father. My mother treated him the same way as me, but he continued to love her blindly even after 7 extramarital affairs of her. I never wanted to happen the same to me, so I never open up to others even if I like them.
But there has to be some genetic part also. I have two younger sisters and they both have some self-esteem and shyness issues. But they have/had normal relationships and close friends and they're definitely not freaks.

Actually in my first post, I meant authoritarian, not authoritative. Pretty big difference.