Part of the problem is that there are multiple layers of "why". There are underlying emotional and psychological reasons and there are conscious rationalizations. Those may shift and evolve as the A progresses from the beginning to the end and then again after discovery.
Right after D-day my WW sent an email to a friend that captured the conscious rationalization that she used to justify cheating during that A. She had changed, I had not. She needed someone who appreciated her for who she really was and allowed her to be that person without feeling judged.
That was the unfiltered version of what she told me at the time. At least what I could get out of her. She was reluctant to talk about it while she was "figuring things out." "Figuring things out," means trying to come up with some reason that might be acceptable to a rational person. Their conscious "why" during the A usually wilts as soon as it is exposed to anyone who isn't an enabling POS.
That's often where the blame-shifting starts if it wasn't already a part of the "whys" during the A. For us it was that she felt unloved and I was depressed and not giving her the attention that she needed. The truth is that I wasn't depressed until she checked out of our marriage.
Finally, eventually, if the WS has a decent therapist they will get into the underlying emotional and psychological "whys." As with so many others for my WW it was FOO issues that caused an inability to self-validate and the need for attention to provide that validation.
And that's absolutely true. I told her dozens of times over the years that the cause of her frequent unhappiness seemed to be her inability to love herself. But it doesn't make any of it feel any better. So often it feels like a get out of jail free card.
Oh,you had FOO issues? Well FOO for me, too!
Because even though I accept that she had real, deep psychological scars that were driving her desire for attention, she wasn't thinking that when she headed out to meet with her AP. "Well, I'm doing this because I have deep psychological need for external validation." No, she gave herself permission because "I've changed and he hasn't, I have a right to be happy."
And that's the kick in the teeth. We expect that someone who has pledged fidelity would pause to question such flimsy reasoning when they are on the verge of such a momentous action. Like, maybe if I'm getting ready to do something against everything I profess to believe in I should take a moment and get some therapy.
That's where the selfishness and the ego come in. Because at that point what they want trumps anything and everything else. And that's what it is, a selfish want.
Even though my WS eventually came to accept responsibility, by that time all of these other versions of "why" had been rug-swept. So I don't feel that we ever fully dealt with those original "whys" or what they said about her mindset at the time. That's on the list for discussion.