The fear of rejection or the fear of acceptance, are good reasons why people may fear going into relationships. Another may be that they may loose their independence.
This is what I fear the most. If I am in a loving relationship, and I have to ask permission to do the things I like, as the other person may not be into the same things. We could all understand the situation when your partner may not like going out clubbing. But there could be other things. My example is that I ride dirt bikes. Some weeks, I take of into the country for the whole weekend, emerging Sunday night, dirty and exhausted.
Now I am sure an understanding partner would let me go out riding, but I like the fact that I can go and do anything I like, when I like, without having to worry about anyone else.
I would love to have a girlfriend that I could do everything I like doing with, but we all know that is pretty hard.
So, I think a fear of loosing your independence is a major reason people may want to stay single.
So, single I will a be, until the right girl comes along, or I settle down a bit. (Yeah right!)