I'm pretty shy and intersting to see this from an extrovert's perspective actually. I don't dislike extroverts in general but their energy, while I find it often enjoyable, overloads my senses a bit. I would never ask anyone to change who they are but if and extrovert is interested in getting me to open up, he will have to turn down the volume a bit.
My shyness is for several main reasons: 1. I am unfortunately, overly sensitive. I don't like this about myself and I have managed to grow in this area but I still take it pretty personally if someone has little respect for my opinions, feelings, thoughts and beliefs. It takes me some time to decide if it's safe for me to open up without being hurt by the person.
2. They are actually not interested in me but in a fun (fun for them) conversation
3.They seem TOO interested in me. This makes me feel pressure to say something brilliant. Not that I'm incapable of brilliance

but pressure to perform freezes me up.
4.I feel we have nothing in common and don't want to waste my energy on a relationship that will go nowhere.
5. You may have actually touched on a topic I'm interested in but I'm afraid if I respond that I will end up stuck in a four hour conversation. I wouldn't fear this so much if it weren't for the fact that so many extroverts that I know aren't very good at recognizing when I've run out of steam. I'm too polite to just bluntly tell them I'm done talking and subtle hints are often ignored.
6. I don't like the person... But you seem like a nice person so that might not be the case.
hope this helps