What could be wrong with me?

I'm usually okay with picking up friends, but hardly ever any closer than acquaintances. And when there is ever a a certain period of distance, say a couple of weeks or so, I find myself detached from even my best acquaintances. Recently, I picked up some very good friends that I love very much, and since we were all at boarding school and had to go home, I've found myself distanced from them as well. The main thing I'm getting at is the simplest things will pull me apart from people, and I can spend every hour of every day with one person, and then our friendship will deteriorate over the matter of a few weeks. As though I wouldn't mind if they weren't in my life anymore. I look at my other friends, and they're still talking to people they haven't seen in years. I don't understand it.