I have been single all my life (I'm in my early 20s). I don't know if I have any desire to be in a relationship with anyone at this time.
It may be selfish, but there are so many things I want to do with my limited time and I feel like a relationship will just hold me back from doing what I want to do.
I guess it is a fear of being tied down to something and impose on my current, practically unlimited, freedom.