I think the search for "why" is eminently more important than the destination of why.
Yes, "wanted to" is a significant component. It's a non-starter without the "wanted to". Where it goes from there -- where that "wanted to" comes from -- is going to be unique to the WS. Figuring that out is some important shit. I will disagree mildly with TG (which doesn't happen often), in that I think many BS's do seem to have a pretty good handle on some of the contextual why's that their WS will land on. I mean, I'd been around my wife's FOO and knew enough of her personal history at D-Day that putting those pieces together upon reflection wasn't exactly rocket science.
That said, accepting the facts of rocket science and accepting the emotional, self-applicable validity of rocket science are separated by a wide, wide gulf. I don't have to figure out how to emotionally own my entire childhood, cSAB, and subsequent relationship dynamics and reinterpret them in light of subsequent knowledge. They're just the facts of my wife's experience to me. They're not vital to me. They're not formative for me. They're just facts. I don't have any work to do to accept, inculcate, or reinterpret them.
That's where the work is.
By the same token, since that work isn't my work...and since I'm not the one who has to do any reinterpreting of my entire formative process to make sense of the A, the destination why's don't really matter. Could be FOO, could be cSAB, could be middle child syndrome, mid-life crisis, mental illness, whatever. Doesn't matter to me as a BS.
Does. Not. Matter.
Except this: if your goal is to reconcile and/or forgive, knowing the why's and trying to see how those lifetime of hurts made so brittle the soul of your spouse is also your window into empathy, into forgiveness, into acceptance and understanding. Understanding their struggle through the lens of their why's is the seed that grows again into love.
(Or, you know, if punishment is your thing, they're also WMD-class ordnance that has just been placed at your disposal. Your spouse's core why is abandonment issues? I don't think you need a manual to exploit that one. Every self-help book that exists for emerging from trauma also happens to be a tactical manual for exploiting statistically significant weakness patterns. Just depends on how you choose to read it.)
For so many BS's, the problem is that the A was a capstone event for us. It was a reality buster. It feels like the end of a quest for destruction -- a culminating event.
But it's not, and especially not in the case of LTA or serial infidelity. We're so busy trying to interpret everything through the lens of this one gamechanging event...and for the WS, it's just another skirmish in a long-standing war of attrition to come to terms with themselves, to heal some old pain, to cope with the struggles of life with the tools they have at their disposal. If it was a gamechanger for them, if it was a culminating event, that instant came when they crossed the threshold into the A, not on D-Day. The lens the BS is looking through is backwards, retroactive, reductive. It hasn't become something that was woven into the fabric of identity. For the BS, it's a full stop, not just another hesitant footstep along the road to somewhere else where the hurt might stop and the world might start making sense, where the emptiness might get filled and the loneliness quenched.
My wife always says that for every "why", you need to ask "why" again at least five more times to get to the underlying cause. I think she's onto something, and I'm thrilled for her (and for us) that she never stopped digging.