Why So Lonely?

Why am I so lonely these past few days? Some of it is probably just the normal upset of having been away too long, and coming back. But still...2 of my roommates are getting married this summer, and so will be moving out, and the other two want different housing (well, one's undecided, but that's kind of how it looks right now).
I don't want to do this again. Every few years, all my friends move away at once, and I have to start all over again. I'm all for reaching out and making new friends, but I don't want this drastic turnover of community anymore. I want friends that will stick around, or at the very least, try to maintain the friendship. I understand that friendships change; but I've never had a steady friend for more than a few years, and I want that to change.
I envy people who have friends they've known since childhood. I have one or two friends from high school that I talk to once a year or so. That's it. And my college friendships are turning out that way, too, and I don't know why. It's never anything drastic, just a slow drifting towards other friends and interests.
I'm just tired of making friends in order to go through the pain of losing them. I don't want to do that anymore.