How?

I am trying to understand how this happens...how do you have a friend one day that says that you are their best friend, they think you are like a sister and then the next day they ignore you and act like they no longer want to be your friend?
I am a loyal friend. Friends until the end. Friends through thick and thin, ups and downs, good times and bad times. I guess I just expected the same in return. I can think of only a few things that somebody could do to me to make me stop being their friend. Things like stealing from me, sleeping with my husband...that sort of stuff. Petty stuff like white lies, small arguments, not agreeing over something etc just does not really bother me.
I would do anything for a friend. I would give them the shirt off my back and my shoes and anything else they needed. I feel like having a friend takes work and is a two way street. If one friend is putting way more into the friendship then the other then it is not an equal friendship.
What saddens me the most is to think that our friendship, which I found very important, seems to be so disposable to the other person. There was no real falling out, no fight, no huge problem that happened to cause this. Just a few things that did not turn out the way my friend wanted them to, which was not my fault, and she has decided to abandon the friendship. I have tried to revive it. Written as if nothing has happened, written and asked what was wrong, written and asked over and over for forgiveness( for what I am not sure because I did not do anything wrong), told her I was sorry for anything that she may think I have done, written and asked about things going on in her life, written and offered to send her goodies and all to no avail. I got an email or two back in reply. Mere sentences that was not much more then you would expect from a stranger, not from somebody who said they cared and loved you.
I am confused . Wondering what I did so wrong in her eyes that would warrant her not wanting to be a part of my life and me a part of hers.
So, this has left me confused and mostly wondering how??? how does this happen.
whew, I needed to get that off my chest lol
It is all ok though, I have a best friend who would never do this to me. I have other friends who would come to me if we had a problem and work it out. This has upset me mostly because I did not do anything and because I just can not figure it out. But I am able to move on, leave the past in the past and only spend the present and the future nurturing friendships that are worthy of more then a quick abandon if at anytime the journey gets a bit bumpy.
I know who my friends are. I want friends who will not ditch me when the going gets tough. That is when a true friend hangs on the tightest. I have spent enough time going over and over in my head what it is I did so wrong and how can I regain this friendship but I realize that it is not my problem. I reached out numerous times and it has not done any good. I will not beg for a friend or spend anymore time thinking about it. I have other more positive things to do with my time and other friends that enjoy my friendship and who want to share things in their life with me and want to hear what is going on in my life.
I am a loyal friend and I guess I expect that in return.
On to a lighter note lol..my best friend is going to be a grandma. I am so excited for her and I can not wait to start helping sew up the baby stuff. Hopefully in the next few weeks they will find out the gender of the baby and then it will be time to start sewing sewing sewing. My other friend is due in august with a baby girl and I am sure I have some pretty girly fabric that is just waiting for me to sew it up into cute stuff.