I think that what makes a friend break-up so painful, is that there is rarely one cause. In a typical relationship, there is usually a point of clarity where you realize it's not meant to be. Maybe someone cheated, maybe the spark is gone, maybe your boyfriend really WAS gay, and you should have listened to your friends about it, but he was so SWEET and the sex was great and.... I digress. With a friendship, however, there isn't usually one event. It's more likely a slow, sad realization that the relationship has changed, and as painful as it is to shut the other person out and move on, you finally see that they haven't been the friend they used to be.
And sometimes you have that one friend, who although the friendship is a shadow of its former self, and you can't even trust the other person anymore, they're still somehow irreplaceable. You hold on, get hurt, and live for those precious few moments when they make you laugh. Everyone around you asks why you don't just walk away, but they don't understand how you cling to the good times in hopes of a return to your glory days.
There is nothing worse than ending a friendship, and it's an extremely difficult decision to make, but I find that if you look into your heart, you almost always know the right answer.