Why do I not care about my friends?

Well I just to know why I don't care about most people. Even my friends, I generally don't care about them in the long run. I feel like I have a lack of motivation to be friends with people or to get a girlfriend, mainly cuz I don't care about them. Don't get me wrong, I am not a pothead and am very motivated in sports. I'm the starting fullback and am close to the 400 record in track. I am enlisting in the marine corps and am motivated to do that, but twice now, when I feel I like a girl, I tell them I don't like them and to not talk to me even though have done nothing wrong. Over spring break I went camping with a girl and got drunk with her and we got very close to ******* but didnt. I liked her at the time but now I just don't really care about her and told her to never talk to me again. Idk why I do this but I've done it twice now and I feel like I'm about to do it again to another girl who is a friend. Is there anything I can do to help this lack of caring for these people?