Attachment Disorder?

Is there a reason why I become unusually sad when people of any degree of closeness to me leave my life? for example, recently one of my co workers moved away. We were friends in work but outside of that we weren't that close. I was sad to the point of crying.

I always thought it was some sort of attachment disorder but now I'm not so sure. Usually it occurs with people who treat me very well or make me feel good about myself. I lost my father when I was about 12 so I also wonder if that has something to do with it. (I'm a female by the way, 20) I know the feeling (maybe a depression) will pass and that it is probably an irrational feeling but that doesn't ease the pain. Maybe knowing the cause of it can help when these feelings begin.