I know how you feel. I used to feel unhappy about not having relationships but if anything had actually happened I would have run a mile. I used to have crushes on people who I would never get, sometimes because they were in relationships, just because I liked the security of knowing that I would never have to deal with an actual relationship. I was older than you are when I finally realised that an actual relationship was something I wanted and that I wanted to make things happen. You will feel ready. Not everyone reaches the same stage at the same time and there's no reason for you to force yourself into conforming to the norm if it's not something you're comfortable with yet.
As for why I felt that way, I'm not completely sure. But I do think that it had a lot to do with how I saw myself and how I felt about myself. I needed to mature and gain confidence and that's probably what you need to do as well. Finally I'm naturally kind of cagey about my feelings and the thought of having to share them with someone else used to frighten me somewhat. Just don't worry about it. There's no race.