Failure
When my friend, who always comes to me to share his / her problems or even to talk to in times of stress, one day goes to someone else or remains alone instead in times of need, that is what I would consider to be the biggest failure in friendship.
This is not because I want to feel important or useful or wanted or anything. It's because I would realize that I might have done something in the relationship where that person is uncomfortable to even open upto me now. I'll constantly be contemplating "what did I do to make him/her think I would not understand her problem?"
Maybe it would be a change in her and not me, maybe her thinking would've changed, not mine... in any case.. i would be effected in a bad way.