I don't have much problem answering the "why" question; but my answer is based on my understanding of the human mind and human consciousness, not on understanding love or relationships.
In a nutshell, I believe that most people expect that other people have some consistent, unchanging (or slowly changing) "true self", and that whatever mask they may be putting on to blend into a particular crowd or social situation, it's still fundamentally "them" underneath. As a corollary, I believe most spouses believe that they know their spouse well enough to have a pretty good handle on who their "true self" is.
I don't think that's accurate. If you look at some of the fascinating research in neuropsychology-- for example, with schizophrenic patients, or with victims of certain types of strokes-- I think there's compelling evidence that our minds are really made of small, specialized subcomponents, that usually map directly to a particular physical area of the brain. In some manner that we do not yet understand, the communication between these various areas gives rise to self-awareness. Also for reasons I haven't seen anyone even attempt to explain, that self-awareness thinks of itself as a single entity, rather than as a collection of output from a bunch of specialized processing units. (Some schizophrenic patients, particularly when taking some medications, are able to clearly distinguish between two or more separate voices inside their brain. Meanwhile, there are many interesting stories about strokes causing damage to only a single area in the brain, with the resulting person thinking and acting in ways that completely defy our intuition about what is core/central to making us who we are.)
Getting back to my original point: most of us expect other people to be very consistent from day to day and situation to situation. This is a terrible blind spot, that causes a lot of suffering in the world. A concrete example is childhood sexual abuse. 90% of children who are sexually abused, are abused by someone they know. For children age 6 and younger, about 50% of the time it's a family member. Even for teenagers, a whopping 25% or so who are abused, are abused by a relative. Why is this possible? I believe the answer is because parents and other authority figures know the abuser, have known them for a long time, and "know" that the abuser is a good person. A loyal friend, a helpful community member, a church leader. A person who has done a lot of good. The abuser is the authority figure's brother or sister, or son or daughter, or-- very common with teenage girls-- the abused teen's step-father. And because the authority figure "knows" the abuser in all of these other contexts, their brain simply refuses to see any warning signs that they might be sexually abusing the child. Their brain actively contrives unlikely scenarios to account for troubling data, to the point where it's automatically done and the person isn't even conscious of doing it. Children go on being sexually abused for years after the abuser would have been spotted, if the people around the child understood that no matter how well they know someone, they only ever know part of them, and they shouldn't assume that part represents who that person is at all times.
It's the same phenomenon as Nazi guards and researchers in concentration camps, who were loving parents and dedicated to taking care of their elderly parents. It's the same factor that leads some men to feel like the old saying, "I want a wife who is a lady in public and a slut in the bedroom".
It's my belief that in real life, no one is a single, unified mono-entity, who has consistent thoughts and consistent values and consistent beliefs all the time. That's an illusion created by our self-awareness. In real life, people can have wildly inconsistent beliefs, not just one day to the next, but one minute to the next. In real life, a person can have complete, utter clarity about how they feel at one time, and then hours later, be equally certain that they feel otherwise-- and they'll be correct both times, because they are made up of many different mental parts, each of which mostly performs its task in its own way, with its own priorities, and then self-awareness somehow tries to make the results all fit together.
A WS can love their BS in the moment, while they're together, and can make statements about the future that they truly believe. Then, hours or days or weeks later, they can see their AP, and talk shit about their BS, and make totally different statements about the future, and they will truly believe every word of it. It's not like having multiple personalities; multiple personalities is like have more than one internally consistent, whole person inhabiting the same body. Rather, it's that the idea of humans being one consistent, relatively static "person" is just false to begin with. We're all a collection of often conflicting thoughts, desires, priorities, etc.
One of the reasons that societies encourage moral codes-- often in the form of religion, but not necessarily so-- is so that its members can apply reason and self restraint in order to act within certain boundaries, regardless of how they're thinking and feeling at the moment. One of the reasons many societies emphasize the sanctity of marriage is because of the long-term benefits, to both children and adults, of placing a barrier to simply choosing to be with whoever you feel like being with at the moment-- if we always wanted to be with the same person, marriage wouldn't serve much purpose.
Anyway, when I read most of the accounts on these forums, I'm usually not too confused by the 'why'. I don't have any trouble imagining a person who is a doting parent and loving spouse part of the time, being a risk-seeking, unempathetic ball of near pure libido at other times. To me, the only question is whether there were ever effective moral/ethical/normative systems in place that kept the person from acting on their inconsistent desires, and if so, why they stopped working.
