Maybe we feel sad because we are left in confusion. If any other type of relationship ends, we know the reason. With this, we really do not know. It is all good today, all bad tonight.
And I guess accepting that confusion is very hard, which is why we are stuck. We are just left with "what/why/how" while they move on as if nothing was a big deal.
I think you have hit the nail on the head here. The questions are still running around in my mind. I really don't have a clue why he suddenly went from totally adoring me and looking forward to the future, to abusing my son and blaming him for all the evil in the world and suddenly running off with the replacement to a much worse life. There is no explanation! That's what really does my head in.