Some friendships die because they aren't moving forward.  They die from stagnation or plain neglect.  You promised to call but didn't.  You knew it was your friend's birthday, but were too busy to celebrate.  When we're busy, we only do what comes easy, and even good friendships aren't always easy.  Lack of attention and concern is sure to cause a rift.  And when it does, it almost always catches us off guard, when we least expect it or can handle it: when we're going through stressful times that make us less attentive and less able to respond - which is what caused the neglect to begin with.  That's why it can seem that the best friendships break precisely when we need them the most.