Why do some friends stop talking to each other?

Well to be honest, I'm still trying to find an answer as to what friendship is really supposed to meant. I was a very loyal friend to a lot of people out there. Always doing as they wanted and however they wanted things to be done, accepting them unconditionally, helping them with whatever I left them believe they need help with. But over time, each and every one of them was taking advantage of this loyalty and expected more and more things to be done with no consideration of my time whatsoever. I felt overwhelmed and pressed, because they brought their pressure to a point where I couldn't even say what I thought was right.

One by one, I had to stab them all in the back to get rid of them and gain my free time back. And in one day, I left all fifty friends of mine in real life. And the confrontations, they were brutal. I had a lot of nightmares for a while after that, but the crisis is over.