A relationship is a smooth analyzable (continuous and continuously differentiable) well behaved function with 2 "hole" discontinuities. The beginning and the end. You know how you got to the beginning and you know where you went after the beginning, you know how you got to the end and and later you will know where you went from the end.
But those moments -- the moment of beginning and the moment of ending -- are insanity. Unanalyzable, discontinuous, not differentiable. Everything goes to infinity or division by zero, the path/graph you were walking on falls out from under you. You just have to keep counting until the function returns to normal and existence proceeds.
ETA (2009/09/12): I was just looking for this post, and before I found it on LJ, I found an old email in which I also characterized the same theory with slightly different words:
Have you heard my "hole in the graph" theory of relationships? That the arc of relationships tends to be uniformly continuous and differentiable at all points EXCEPT at the moment of getting together and at the moment of breaking up. You see where it's heading, you see where you've been, you see what's likely to come....but even if you see being together or being apart as the logical outcome of the current course of things, everything still falls apart at those moments of transition. You have no data. Math breaks down. Prediction breaks down. Understanding of how you got there or why breaks down? It's chaos. It's change. It's intense. And later in retrospect, if you ignore the hole in the graph, it all seems to make so much sense. But if you look at that hole you get lost again.