This is something I think about a lot, and have written about from many angles.... for years, for most of this crisis I have wondered what on earth could have been missing for my H, as he had parents who loved him and showed it.
My MIL wasn't a cold and uncaring person. But perhaps his being the apple of her eye had another side. My MIL, despite being affectionate and always having dinner on the table, etc., perhaps wasn't able to do the taking care of in other ways. For instance, she never learned to drive, so always depended on FIL (or perhaps for a short time before he went to university, H) to drive her place. Of course she used the buses well, but still. That's just one example.
So in some ways MIL needed taking care of herself, and perhaps that came through in that she wasn't able to take care of H more. Not that she didn't take care of H -- that is why this seems so convoluted to me. Perhaps he just needed, or THOUGHT he needed, more than she could give. She wouldn't have been the one to exhaustively research educational things, for example, of one of her children needed something special.
I'm the opposite -- I'm the super-fixer. Medical problem? I'll know more than the doctors before long. And so on. Kids' education? I'll look at more schools than anyone else. I'm overdoing it a bit here, but I think you know what I mean. I'm not helpless; she, despite being loving and kind, was always a bit so. Or at least I got that impression.
I may be grasping at straws here; I know this is a big part of it for H, just still not sure exactly how. I know that once, in the first year after he left, when I was very ill, he told me that I had become ill because I wanted him to take care of me. So somewhere there he was rebelling against that bit, I guess he wanted to be taken care of more, but of course he won't let me do so.
I have no idea what this OW is like so I don't know where she fits on this scale; does she do that kind of taking care of? Is that the idea, is that what they are looking for? I can see where I and the kids just represent things HE needs to do to take care of people; does she ask for nothing? I see I'm not making a lot of sense here; I'm working this out in writing.
Anyway, very interesting topic.