Funny that this should come up today - it's exactly three years since my last relationship ended.
I'm single because I prefer it that way. I've been there, done the whole relationship thing and have no interest in doing it again in the immediate future. I value my personal time and space over absolutely everything else and I get really irritated if anyone gets in the way of that. I like to do what I want, when I want to do it, without anyone interfering. I'm far from a loner, though. I love spending time with people but only when I want to, which isn't really conducive to maintaining a relationship - in fact, it's barely even conducive to living with housemates, particularly if they're the sort of people who'll insist on hanging out just because they're bored, as if being in the same room as another person will somehow alleviate that boredom. (That said, I've been lucky with housemates, they've all been generally lovely and quite accepting of the whole 'personal space' thing.)