OP, I have had the same problem (it's not over yet, but I think I'm finally making some inroads). I've concluded that basically, people will stick around in your life as long as they are receiving something from you that they want. That can be genuine connection, it can be some sort of cachet or "coolness" from knowing you, it can be money or job-related, it can be as simple as needing someone to hang out with... Which one, or what combination of the above, depends on the two people involved.

I find that really connecting with people and becoming emotionally involved is a good guarantee that they will stick around. Unfortunately, I've observed that more than actually done it myself. Working on it.

Hopefully this isn't reinventing the wheel, but it actually took me a long time to figure this out.