I think it happens for a few reasons, but it’s gets back to how the deception of an affair begins in a persons mind until that deception leads to making choices they wouldn’t have otherwise made. In order to have an affair and maintain it, they have to alter the story or memories of their marriage to be more negative than it was. This is how an, otherwise moral and high values individual can make choices that led them into having an affair.
Once we change the tapes of our history, and amplify the problems we had in our marriage, it becomes easier to allow ourselves the “right” to have an affair. Compartmentalizing the affair often becomes common and this closes off feelings about their spouse, which would likely create enough guilt to cause them to want to end the affair. Justifications and changing the real history of the marriage is a dangerous and slippery slope, and it all happened one compromise at a time.