Fear of social rejection
Fear that one isn't adequate or interesting enough
Fear that one's past experiences or hobbies are embarrassing
I think I have social anxiety disorder and I find it extremely difficult to start a conversation with a new crowd, but I am alright with continuing a conversation. I started a new job working in an office with 10 or so people and I was incredibly shy at first; so much so that one of the people actually came up to me and said it was concerning the other workers that I (basically) ignored them. More recently I have been friendlier and more vocal with them and I am now comfortable with having a light conversation.
For me, I'm terrified of those first few moments / encouters with new people. I am also terrified of meeting those that I went to school with (I'm in my mid 20s now), because that time of my life is when I probably regressed into being a social recluse, whilst everyone else was having fun!
Oh well...