I want to offer my thoughts on the questions you pose. I have had a lot of years to ponder those same questions.
Background: My W,for a variety of reasons, dropped out of a professional school graducate program even though she was at the top of her class. That made her feel depressed and lost. OM, who had been her classmate and hot for her, contacted her to "see how she was doing" and to "cheer her up". You know how that turned out.
So she has told me that in addition to being depressed and lost, she was bored with life; looking to have an adventure; wanting to have some excitement. And OM came along and said all the right things and she chose to make an A with him her adventure and her excitement.
So to your question of can we ever really understand "why". I don't think you will ever be able to understand "why" in the way you want to understand it. You want to understand it in a way that makes sense to your brain; in a way that makes you not feel so demeaned and worthless and allows some dignity and self respect; that causes you to feel it really wasn't such a big deal to her and that she really means it when she says he doesn't match up to you.
The truth is, he didn't match up to you. He was a pointless diversion to her - nothing more. But the problem with our brains is that they keep going back to the thought process that: she did it; she wasn't drunk; it was a deliberate, planned out action, perhaps many times over many months; and so it must have been special, he must have been better and yada yada yada.
You are human. Your brain cannot make sense of the A in any way other than thinking it/he must have been special.
I get that. So what you have to work on is two things. First, understand that her "why" was not rational. It may have seemed rational to her at the time, but only because she was thinking irrationally. And you see, if you could somehow put your brain into "irrational mode" and, while in that mode, analyze her actions and her thoughts -- you would then understand her "why". But I suspect you can't do that. I have never been able to. I can try to step back, outside myself, and look at it and intellectually I can see what she is saying. But then the rational comes rushing back in and says "but I would have been happy to be the focus of your adventure, etc".
So I suggest that you listen to what she can tell you but don't expect that it will make sense to you as a rational, reasonable "why".
Secondly, don't let the fact that you can't understand "why" derail your healing or your work at Reconciliation. Accept that you will never understand her why and instead focus on whether she is now back to being the woman that you married and wanted to be your Wife. If she is; if she is doing the right things, saying the right things -- let the "why" go. Begin building the future. The future can be awesome.
You also ask if it is too much to expect your W to feel the same pain you have felt.
Yes, it is. You can never understand her why. She can never really understand your pain. She hasn't BEEN there. How could she feel it. Only a betrayed spouse can know the depth and breadth and agony of pain caused by such a betrayal. So your W will never fully understand your pain. And frankly, I wouldn't want my W to ever feel the pain I have been through. I still love her. Deeply. So there is no way I would wish that kind of pain on her.
When did I know reconciliation was possible? Well I lived for many years still loving her and finding times of happiness and joy amongst the times of pain. But we never talked about the A because she refused to. Just refused -- and I quite unwisely failed to insist and force conversation. A reconcilliation of sorts, but not true, meaningful R.
True R, for me, began to occur when we finally started talking. When I could ask questions and get answers. When I began to learn which of my assumptions about the A
were accurate and which weren't. And that is when I came to know, and understand, what true R is and that it was occurring.
I hope you and she can and do talk freely and often. And that she is open and honest in her answers. That, and continued efforts by her to help you heal and regain your sense of worth, will get you where you need to be.
Peace to you.