Hi Manda,
I've often wondered the same and your post does make sense to me btw.

Over the years I've had a myriad of "friend" go in and out of my life, some I knew for many years and now really have nothing to do with them and really only 1 person I've known for the last 16 years and even we're not as close as we once were. The thing is I think I'm beginning to learn for myself (though it may very well be wrong) is that people come in and out of your life as needed to teach us lessons. Sometimes they stay around for a while and sometimes it seems that the time with them just flies by.
Maintaining a friendship is a two-way street just like any relationship. We can try as hard as possible but if the other person doesn't want or see us like that then they'll not respond as we think they should and/or leave. It's hard and devastating at times and you feel so betrayed but it seems to be the way it is.
I've not real words of wisdom but maybe my post helps seeing the answer to your question in one particular light.
I hope you have a very nice day.
Chris