I'm generally considered to be shy and I certainly acted shy for a lot of my life.
But it wasn't how things always were or always are. I guess deep down I'm just a weird person so when I act in an outgoing way (my natural personality) I get constantly cut down and insulted. So I repress my behavior. Simple as that.
When I was a kid I just didn't care when people insulted me because I wasn't very self-conscious, I wouldn't realize what was going on. But once I got old enough to notice that people were constantly crapping all over me I started to quiet down.
If I'm around someone I know very well I'll open up and be myself, but there are very very few people in the world who can tolerate my natural personality without constantly insulting and degrading me. Even people who have known me for 15 years don't like my true personality and will respond poorly when I let it out. So I basically have a bunch of fake personalities that I use depending on who I'm talking to. This means that social interaction is extremely draining because I constantly am faking. I'm faking right now. Since social interaction is draining I eventually just start to not participate because it's no fun for me, hence the "shy" label.
I'm just one of those people who people hate. It's not like my true personality has ever changed - my true personality has been the same since I was 3 years old. But I've intentionally repressed it because when I let it out, I immediately regret it due to the ridicule.
BTW a hell of a lot of nerdy guys find shy girls cute. It's not really considered a negative trait. Shyness for a guy though is social suicide. You can see that in this thread for instance, all the guys are into Bliss Sage the shy girl
Shy girls = cute. Shy guys = creepy/feminine.