Like Thorne, I'm comfortable by myself. To me, being solitary does not equate with being lonely. Loneliness tends to strike me when I have the expectation to be able to share and to communicate, and I am struck with the realization that I don't have a receptive person with which to do so.
For example, I used to have a relationship with a man in the pharmaceutical field. There is big money in medicine of course, and as a result these folks both threw and attended lavish conventions all over the place - art museums, historical centers, film openings. I had some wonderful opportunities to go to some amazing places. I looked forward to these events at first, but sometimes it seemed I was one of the very few people in the room that was interested in the museum or the reason for the fundraiser and so on. Maybe it was that a lot of these folks had "been there done that," maybe it was the fact that I was typically 20 years younger than the rest of the room, but whatever the reason I always felt extremely lonely when I realized I was happy and having a good time without any real outlet to share. That's lonely for me.