Your therapist will guide you through a series of exercises to answer the "why" questions. Please view all of the items below as EXPLANATIONS, not JUSTIFICATIONS.
Here are some the common contributing factors to an affair which may be considered by the couple:
FAMILY OF ORIGIN
Attachment style: Attachment styles, such as attachment avoidance or attachment insecurity, determine how we relate (or don't relate) to others. Looking at how we "attach" can provide insight to why someone has an affair.
Expectations from Childhood: The way a person is raised sets expectations for about almost everything in a marriage, including how to treat a spouse, parenting, money and sex. Examining one's family of origin can provide real insights into how an affair happened.
Childhood trauma: Having a history of childhood trauma (such as physical, sexual, or emotional abuse or neglect) may play a significant factor in infidelity, especially if it has not been previously addressed.
Exposure to infidelity in childhood: Having a parent who was unfaithful increases the risk of an affair. In fact, children who are exposed to a parent having an affair are twice as likely to have an affair themselves.
Communication style: Families communicate is radically different ways. If one family is loud and embraces conflict and the other is quiet and avoids it, a huge gulf can develop in the relationship leaving room for an affair.
Honesty: Some families have a loose relationship with the truth. It may be OK to lie because "everybody does it". Consider how members of your family of origin responded to a lie and how those examples influenced your behaviors in the affair. This is of major importance as the lies covering up the infidelity is often as damaging to the marriage as the infidelity itself.
Mood disorders: Bipolar disorder or clinical depression may impact a person's ability to see the marriage or life in constructive ways and thus can send a couple on a path to infidelity.
Addiction: Addiction to alcohol, drugs, gambling, or something else, are often contributors to infidelity. Substances may reduce inhibitions allowing a person to cross a line that normal would be respected.
Psychological Makeup: Unfaithful partners with traits of narcissism (ego-driven, entitlement, self-centered...) may lead to a way of interacting that can lead to betrayal of a loved one. Other personality disorders may also be considered as appropriate.
Body image/aging: This happens with both genders but most often we see middle-aged men having an affair with significantly younger women. The infidelity may help the unfaithful partner to prove that they still are wanted physically. Also, a spouse may blame the affair on the spouse by claiming that he/she has "let himself/herself go."
CONTRIBUTING MARITAL FACTORS
Unhappiness/Dissatisfaction: Dissatisfaction often results from neglect or mistreatment within the marriage. This usually gets expressed as feeling emotionally or sexually dissatisfied or disconnected. Couples must nurture the relationship or else the partners will drift apart.
Sexless marriage: A sexless marriage is often claimed as a reason for both men and women. The reasons for the lack of absence of sex must be understood.
Feeling unappreciated: Partners who report feeling undervalued or neglected are more likely to have an affair. Women often carry the bulk of the housework and childcare, even when both partners are fully employed. Having an affair can make one feel wanted and appreciated. Nonetheless, the couple should be mindful that feelings of neglect may be linked to unrealistic expectations.
Lack of commitment: Some long-term relationships, including marriage were never built on a true commitment to each other. How the relationship came to be should be thoroughly and honestly discussed.
Boredom: Keeping a marriage fresh takes work. Also, a couple needs to understand how to transition to a more mature (and deeper) way to love each other. In the absence of such efforts, a partner may be pulled into the thrill of the chase and the high of a newfound love.
These factors address how the affair happened more than why they it happened but they are definitely worth discussing:
Internet: Having a cyber affair is now incredibly easy to pull off. Also, going from a cyber affair to a physical one is common. The role of the Internet should be considered in order to fully understand how the couple arrived at the point of betrayal.
Opportunity: Long periods of travel or serving in the military may compound the primary factors that explain an affair. The belief that an infidelity will not be discovered may be a factor. Also Feelings of abandonment and/or loneliness may tip the scales toward infidelity.
Poor boundaries: Some people have never learned wiser personal boundaries. "People pleasers" may put themselves at constant risk by not knowing how to say no to those who make advances.
Pornography: Children are exposed to pornography at increasingly young ages. This may distort a partner's sexual development and impact the ability to bond sexually with an intimate partner.
EVEN MORE REASONS
As if the above list of reason for an affair is not long enough, we have included a summary by Glass and Wright
(1992) which was derived from a review of publications about affair. The list included:
1) sexual attraction,
2) gratification of unmet sexual needs,
3) gratification of unmet psychological needs (need to be mothered, fathered, nurtured),
4) gratification of unmet social needs (mating with someone of a higher social status),
5) as a bridge to escape an unsatisfactory marriage (dependent person with or without
6) to fulfill a need to conquer or to dominate the opposite sex,
7) for power or control issues,
8) to fulfill a need for love or to culminate an already loving relationship,
9) to fulfill extraordinary sexual drive or sexual compulsion,
10) for purposes of revenge,
11) as a result of alcohol, drug related, or other impulse control problems or disorders,
12) as a result of liberal sexual values, and
13) as a result of opportunity (e.g., job-related travel, physical separation from a spouse, or frequent contact with potential partners).
PRE-INFIDELITY VERSUS POST-INFIDELITY ISSUES
Usually unfaithful partners want to talk about issues that existed before the affair or and betrayed partners focus on issues that occurred after the affair.
For example, the unfaithful partner will zero in on how he/she was always criticized and the betrayed partner will ruminate on all the dishonesty that happened during the last year of the affair. When the couple is trying to learn from the affair, all factors should be discussed and taken into consideration as the couple is trying to figure out how the got to this place. It is not a question of "either-or" but rather a "both-and" learning exercise.
We sincerely hope that you are not overwhelmed by everything list above but rather that you find the reasons that resonate with you for your relationship and explore them with curiosity and compassion within the safe confines of therapy.
Glass, S. P., & Wright, T. L. (1992). Justifications for extramarital relationships: The association between attitudes, behaviors, and gender. Journal of Sex Research, 29, 361–387.