Lying is one of the more fascinating behaviors that I have studied.
There are many reasons people lie. Basics remain the same, across cultures, and across languages.
1. To get out of trouble.
2. To make oneself appear grander as a person.
3. To cover shameful behavior.
4. To cause others to perform in a way that results in gain for one's self (either legal or illegal activities).
5. To obtain goods or services in less than standard or normal routes or means (i.e., blackmarket, underground, illicit and/or legal/illegal goods or services).
Those are kind of the basics. Of course, there are many other reasons for lying.
Lying includes lies of comission and omission. Deceptive speech, which is speech that purposely misleads but does not particularly "state" something, is also a fascinating area of study. There is one poster who used to be here who was very skilled in this area, and quite populer. Doesn't post here much anymore. But when that person did post, I always found it quite interesting, because the post often said NOTHING, and many people agreed!
Yet when analyzed, that person's post would be nonsense.
There are many politicians who do this, too!
You ask why your WS lies.
Fact is, it is instictive. Children who have just begun to speak have an instinctive lying response when they know they are in trouble. They are TERRIBLE liars, but do it anyway. Researchers have looked at this, and have found children as young as 20 months old who lie. For example, they will eat something when told not to, and then lie and say they did not eat it.
There is also the issue of lying due to self-deception. Self-deception is an interesting human behavior. Initially, when self-deception is happening, the person is actually not aware of it (otherwise, they could not deceive themselves). Once the person does become aware, they begin to lie because they are aware of the betrayal of their own conscience. This results in a human reaction to find someone to blame.
That person, in the case of an affair - is the spouse. Why, of course the spouse is to blame!!!! The spouse is, is, is,,,,,let's see.....that BS was NEVER what I wanted, that's it!
That's when the marital history begins to be rewritten. That point of realization of betrayal, the WS's brain goes into this mode of finding someone blameworthy. That's when the lies start, the blaming starts, and the justifying starts.
A person doesn't do things that are so wrong without some sort of justification happening inside the mentation system. And this system is well-defined and well-organized in humans.
For a good understanding of how we ALL do this, read:
Leadership and Self-Betrayal: Getting out of the Box
by The Arbinger Institute
It will also help you a great deal on your Plan A.
SB