As I look back over the other best friendships I've had that also ended, I wonder if, in addition to simply having a finite amount of time for such intimacy, we also have certain periods in our lives in which we seek out people who seem to embody the things we lack. Then, when we gain those things for ourselves, we no longer need that friend in the same way, which causes a serious dissonance in the relationship. Perhaps this is why these particular friendships burn so bright and then disappear so completely.