I'm single because:
I am very, very, very picky. I figure I'd rather be single than be stuck in a relationship with someone I'm not totally crazy about.
Unfortunately, the few who pass my standards do not seem to like me that way.
I'm lazy. I don't make much of an effort to meet people and get to know mildly interesting people.
I'm busy. I work a lot. Also, I do a lot of other things when I'm not at work and most of them do not involve meeting single men.
I'm shy. I will not make the first move because rejection sucks.
I take relationships seriously, that is something I can't help, so I become pretty emotionally involved. This results in me being extremely picky. Also, because relationships are so important to me when I have one, it's not really compatible with my being extremely busy at this younger stage in my life.
Or maybe just because I'm young and don't really know what I am looking for, and thus am commitment-phobic.