I don't know what to tell you, but based on what I've been dealing with and what spouses who have separated for extended periods have told me, "they really don't know". Two I've talked to this summer left their husbands for over a year. They felt overwhelmed by what on the surface seems like a large pile of little things that to most logical ( read sane)mind seemed like no reason to leave, a situation that could have been avoided by talking about it instead of internalizing it.
They appear to just get overwhelmed by it all,lock the good parts of their relationship in "hate boxes" and look for anything that diverts their attention from the commitments they can no longer handle,EG. marriage, kids,family. I really think it's a form of nervous breakdown that triggers our primitive "fight or Flight" syndrome, they leave before they totally lose it.
This is doubly strange to us because of what we see as the selfishness of this action. The stuff they could not handle they leave us with double the load we had, and walk away. Coming back from this must be almost as bad as their lead up to leaving, I know if I had done what my W has to me and our Family, the guilt and shame of what I had done would be overpowering.
