By choice-ish.
I don't really have an interest in a deeper relationship with anyone. I've always had a close knit group of friends, and I've always been content with it. It gives me the ability to have social opportunities, yet also lets me still enjoy my own quiet time when I need it, without having to worry about a significant other being upset that I don't want to spend time with them at the moment. Friends always seem to have had that problem when I watch their relationships, and they voice it as well, that their other always wants to be with them. And that doesn't sit right with me.
I also will submit that I'm socially awkward. I can convey better in text than what I can in face to face. I always end up standing too close to someone, or stumbling over my words as I try to tell a joke, or something along those lines. Try as I might, when meeting new people, most turn away from wanting to converse more because "He's too shy." (Which is true.) The few that see past it and stick with me, I'm perfectly happy to have on my side. And I'll even let them know it. Eventually social situations will click for me more, I'm sure, but at the moment they only serve to enforce my choice of staying single.
In the end, though, it's by choice. There's nothing stopping me from stepping up and giving it my best shot. People are going to have their opinions about everyone, and there's nothing I can do to stop that, so why not try? But at the same time I'm as happy as I've ever been, and I'm flying solo. And that's great enough for me.