Have you ever wondered if you're just too independent to be in a relationship? You're in control of your life and you don't need anyone coming in and missing it up. You like order and structure in your life. You can easily get stressed if things aren't in order and you have to spend extra time getting things back in order. You would enjoy the company of someone but at the same time you don't want the heavy demands of being in a relationship. Plus how much of your life will then change. Will this person get in the way of achieving life goals? What if everything that you've been working so hard to achieve gets taken away by this person? What would you have to change for them to make them happy; will that mean that you'll end up unhappy?
If this sounds like you than you also know how you've been hurt by someone special in your life. You've built up safe walls around you to protect you from getting hurt again. Maybe it's not by an actual relationship but it's been by someone that has rejected you; sister, friend, teacher, etc. It could be a dad who was never around for you and you always felt that you weren't good enough to have someone's love. You then look at every relationship as someone who won't love you back.
It's part of being human to have these types of feelings. They are coping mechanisms. You should also realize that most of the time these ways of coping aren't typically healthy. Go day to day wondering if the next person you meet is going to hurt you isn't the best way to live your life. No one can guarantee that you'll never be hurt again or be rejected by someone. It just means that you need to deal with those hurt feelings and then let go of that hurt. You're no longer allowing those hurts to hold power over you and your relationships. Those hurts aren't serving any purpose in your life. It's good to use caution when getting into a relationship but you should also keep an open mind and let someone surprise you and love you back.