Why is it hard for me to be in a relationship?

When a girl first meets me I am shy, but once she gets to know me I am very outgoing. I'm 22 and only have been in a few relationships. My friends seem to always be in a relationship while I am known as the single guy in the group. Some days it bugs me always being the single guy, and other days I could care less. I'm a caring guy who sticks up for what he believes. Since I'm so used to being single, I always do my own thing. My schedule revolves around working out, school, and work. I have jumped a little fast and ended up scaring girls away. I've also taken things slow and the girl ended up running away. Maybe I think different from my age group. Sometimes I tend to over think situations. I'm straight forward with how I feel.Sometimes I don't think about what I'm going to say and I appear as an idiot to some people, but I do make great grades in school. I may not be the sharpest tool in the shed, but I can laugh at myself and move on with my life. I cannot help that I am really gullible and act ditsy at times. I'm a kind and caring guy, but I'm okay with that. I don't have to be an a*shole to prove myself. If it means I have to be an a*shole in order to be in a relationship, then I am okay with being single.