Maybe you just aren't ready to be in a committed relationship. You might need more time to find yourself first. Nothing wrong with that..Everyone is different in their comfort level with others in intimate situations, worse thing you can do is force yourself to be something you aren't.
What are you worried about the most?
Is there a specific person you have in mind already?
Are you young? Because when I was young I used to be this way. Really like someone and then when they really liked me I didn't like them any more. Part of it is because you are not ready for committment or maybe the excitement or thrill of the chase is gone once you get them. It does change when you get a little older but sometimes you may think you like someone and then once you get to know them better, you realize there is no chemistry or they aren't who you thought they were. Once you meet that man that is the one, you will know. Doesn't mean that you will stay together forever but it means that there is something there that makes you want to involve yourself in a relationship. Don't rush it, it will happen when it's the right time.