I think most of the people in this thread (including myself) can related to this article:

http://en.wikipedia.org/wiki/Schizoid_personality_disorder

Now, I think calling it a "disorder" is rather harsh since we are all very functioning individuals, but this is undoubtedly a "condition" we all have.

The part of the article on the "secret schizoid" describes myself and many others in this thread perfectly. We can be very social when necessary and even have a really interesting and interactive personality, but in the end we prefer genuine solitude to relationships with others. Definitely read the "secret schizoid" section of the article too.