Curiosity. Maybe a faint hope to find and meet someone exceptional. But to be honest, I like being alone most of the time, I like doing stuff alone, and I will not compromise that for the sake of it. I know enough people who are only in relationships because they are afraid of being alone. That's not for me.
When I think back of past relationships, some of my happiest memories are coming back home, alone, and thinking "well well well, now that's more like it".