For me the Why? was important because we were attempting reconciliation.
It was necessary tor me before even beginning to rebuild any measure of trust to know that HE understood the WHY. That he could figure our why, after 25 years of being faithful and loving with many, many opportunities to cheat that were not seized (which for me, defies the "just because they could" philosophy) that at this particular time and place he felt compelled and entitled to blow up our marriage and hide things that affected my health and well-being from me.
I needed HIM to understand it even more than I needed to, because the only one that could truly police (and change) his behavior was him. And if he didn't understand why he did the things he did, he was doomed to repeat them. But if he did - then he could look out for the kind of thinking and feeling that got him into trouble and deal with them early (similar to how successful recovered addicts deal with their addiction.)
Sometimes even when we understand the why (for any behavior - drugs, sex, lying, overeating, manipulating, controlling, etc.) we still choose to repeat bad choices. And at that point it becomes a conscious choice rather than an unconscious emotional reaction. But if we are motivated by a desire to change, grow, do better and be better, that self knowledge is the critical first step in a series that allows you to take control of who you are and who you become. It isn't the only step so the WHY alone won't get you there, but it is all-important in the kind of "emotional growing-up" these people need to do.
On the other hand, if you are getting a divorce I am not sure if the WHY is necessary. I do think understanding helps in the healing, but isn't necessary.
Crushed - I think it is likely best that you are getting a divorce. It seems to me your husband's answers are one of two things: 1) a lie because he doesn't want to delve into the deeper reasons for why he behaves as he does so he just throws out an answer to "placate" you or 2) the truth, which means he's been a liar since the day you met him. He knew as he was taking his vows he didn't believe that he needed to forsake all others and that sex wasn't a big deal. Neither makes him sound like a good bet for the future.
Crushed - I know it feels like all his reasons/excuses say something about your worth or value. If I was lovable enough how could he treat me this way? If I am valuable, why would he take the chance of losing me? But our value has NOTHING to do with their actions. Men married to successful, rich, breathtakingly beautiful women still cheat - and many who are married to remarkably average underachievers don't. Because how you act and what you do is about who YOU are deep inside. Whether you believe this world is an “eat or be eaten” world where only suckers play by the rules. Whether you derive satisfaction from being a person of your word or prefer to get your every need met the second you have it regardless of consequences. Whether you believe that there is value in investing your energy and time into other people, even when it is hard and asking/expecting them to do the same.
Your husband is the one that should be questioning his worth - not you. The only questions you need to be asking yourself is what signs did you not see or pay attention to along that way that should have warned you that he was not a man of integrity? And how can you see people more clearly going forward so you know who to invest time/care in - and who to stay the heck away from.
His inability to value you and your marriage properly says NOTHING about you and everything about him.
Vanessa - I agree with a lot of what you have said, with the exception of the why being because they "could." I could have had so many affairs by this point in my life, we'd be able to play "7 degrees of Thrive" by now. My husband could have as well. We are both attractive people who worked long hours at jobs we were passionate about with large groups of other attractive, interesting people. Opportunities were plentiful.
Neither of us are particularly jealous, so we each had a lot of freedom of movement and time that would have easily allowed for an affair to bloom and be hidden. But neither of us did for 25 years (and I was tempted at times when I felt disconnected to him emotionally, but never acted on it). Then, when a very specific set of experiences and situations took place over 3-5 years that built up of well of resentment, hurt and anger followed by an opportunity similar to many that had been rejected in the past arose - he took it. Around the same time, I had a similar opportunity, and while tempted, choose not to. Opportunity does not equal an affair. Lack of willingness to deal with your feelings and needs in an open and honest way leads to affairs (in my opinion.)
If I misinterpreted your meaning and in saying that they "could" you meant because there was something missing in them, something that made them choose to not consider the harm to those they love, their own self-image, etc. than I would agree. But then, in my mind, that is still part of the WHY? Why is that part missing? What has prevented them from learning the kind of empathy and compassion that would provide the consideration of the consequences to yourself and others?
For me one of the biggest indicators of whether there is any chance for successful reconciliation is the WS's HONEST exploration of the WHYs - followed by a plan of action to address them and change their learned responses. And the Why's have to be about them - not their partner.
If a why is "I felt rejected when you worked so much" - the exploration needs to be about why they interpreted it to mean they didn't matter, and why they didn't use more appropriate avenues to address it (talking, counseling, compromise, etc.) If the Why's are a laundry list of their partner's faults - it isn't an honest exploration, it's just blame shifting.
Anyway, after wayyyyyy too much time thinking abotu this, it is where I ended up. Might not make sense to anyone else, but it made sense to me.