As a kid sometimes I thought I was shy, but it always made no sense to me, because I didn't really feel that way. The reason why I thought that, because I couldn't do anything I wanted, like in a dream, for instance. Subconsciously I knew, that maybe, it's not a good idea
but consciously I blamed myself for not being able to do so. Now I know what was happening to me, back then I only partially realized it.
1) when I imagined what I was going to say, I imagined it in pictures not in words. So when I opened my mouth, sometimes I couldn't really say anything reasonable because I just couldn't put the right words in a sentence. People weren't usually patient, and didn't want to wait for a half an hour for me to give birth to some speech
2) in many cases I couldn't predict other people reactions, when i got an unexpected reaction I couldn't respond to it, because I wasn't prepared and my mind would just go blank.
3) I didnt really know how to socialize, what to say. How to start a conversation. When I was a kid I would usually just tell stories, or come up with new games. I didn't know what to do when it didn't seem appropriate to tell stories etc.
I learned how to socialize in high school (by reading books about it), it was going pretty well but I still had hard time predicting other people's actions. I'm better at it now, because I'm aware of the issue, so I am more attentive and analyze people more to identify their possible behavior. Before when I would suddenly decided, "hell with it, I'll do whatever I want" - I usually didn't get reactions I expected and sometime got myself in a little bit of a trouble, usually I made those uncomfortable situations into a joke and put it in my "oh, crap!" adventures list
so... I don't know, sometimes I wonder do "shy" people really exist?