Has anyone ever stopped being your friend, but you didn't know why?

I'm at a stage in my life where I really don't need any more friends. So, if a pal totally goes MIA on me, or lets me down or doesn't return my calls, I really don't stress about it too much. That's not to say that my feelings don't get hurt, but I just don't have the energy to keep making the effort with that person anymore. Helllooo, I have enough shit to worry about right now! (Have I mentioned that after today I will be jobless once again? Back to the drawing board!)

I've had a handful of close friends over my lifetime that just stopped being my friend. (And I don't just mean that we lost touch. I've lost touch with people that I'd still consider a friend and vice versa--this is different). There was no call or email with an explanation--it just...happened. A couple of them had issues within themselves that always kinda prevented them from being good friends, so that was easier for me to accept. But there have been others that I took a little more personally. I guess I just think that if you care enough about someone, you'll let them know if they hurt you, even if it's extremely hard to do. I've never done something blatant or intentional to hurt a friend, but I do think there might have been a time or two where something happened that was misinterpreted.

I know I'm being incredibly vague here, but honestly, if any of the people I'm thinking of knew I was referring to them, I'd be a little embarrassed. I doubt they ever think about me and wish things were back to how they used to be in the good ol' days. Ouch, my heart is feeling heavy just from writing this. Ohh well, it's good to get it out. Thanks for listening :)

What about you: Has anyone ever stopped being you friend, but didn't know why? What happened? Does it bother you still?