Why do people cheat in a relationship?
Why would someone want to hurt you so much by being unfaithful?
Here you'll find a list of reasons why someone might cheat.
None of them are excuses, of course! The reasons in this list are circumstances leading up to someone - a partner or spouse - having an affair. They may offer an explanation of sorts, but they are not the cause of a person's infidelity.
The unfaithful person cannot blame their partner for the affair because the infidelity is the result of a course of action they have chosen. Their partner may have played a role in what occurred, though.
Reasons why someone might cheat
I have given 20 reasons here, but you'll probably notice that some of them overlap. As an experienced couples therapist, I've found that the wronged partner is often desperate to know why their partner or spouse cheated.
However, the adulterer him or herself is seldom able or willing to give a coherent answer. This is sometimes because remorse stops them from wanting to hurt their partner any further. But more often than not it's simply because they have little sense themselves of how or why the affair began and continued. They may say something like: "It just happened."
So, here are some potential underlying reasons for infidelity...
20 reasons why people cheat
1. Major life transitions
Transitions such as pregnancy (see: Pregnant and stressed by my spouse), birth (see: Overcoming a traumatic birth), children leaving home, mid-life crisis (see also: perimenopause symptoms) etc. require a huge emotional investment.
They always involve an ending of some kind which can provoke a sense of joy, hope and/or dread. Feelings of insecurity can also present themselves during these times. Infidelity can be an escape - a way to avoid facing up to the reality of the situation.
2. Specific relationship issues
Intractable relationship problems or a relationship crisis can be another reason why someone might want to escape.
It may be that one partner feels that their needs and wants just aren't being met. Their expectations may be too high. So, they stray in search of someone else who can meet their needs and wants.
3. Issues around identity
Acting out a sexuality or gender and/or living in a body that feels wrong can lead many people to adopt a life that they think is in line with other's expectations.
Many follow the traditional heterosexual marriage path in the hope that everything will somehow work itself out. They may be hiding their true identity not only from others but also from themselves (see: My partner is bisexual). Ultimately, they feel so miserable that they embark on illicit affairs. Often this is to test what it would be like to be in a relationship with someone of their preferred gender.
This is when one partner is determined to play the field. They are searching for an easy target. They have the ability to build rapport with others quickly and overall they possess a feeling of entitlement.
5. Obsessive Compulsive Disorder (OCD)
For information about what that entails, see this article on verywellmind.com
6. A history of infidelity in the family, e.g. parents’ extramarital affairs
The study Family Background and Propensity to Engage in Infidelity by Weiser et al. showed a link between parental infidelity and their children's inclination for infidelity .
7. Specific escape from distressing situations, e.g. infertility, illness, disability, unemployment, etc
This involves the partner seeking something to make them feel better. The attention of someone else fills this need.
8. A less than satisfying sexual - or even sexless - relationship
The utter frustration and the degree of powerlessness that is felt when one partner appears to hold all the cards leads some to stray into a (sexual) affair. See also: What you can do when you have a sexless marriage.
A sense of status is an essential emotional need. As human beings we need to feel like we have a place in this world and that we're seen and accepted for who we really are. Being seen as worthwhile by someone who isn't their partner may be a way to fulfil that need.
Lack of impulse control or plain selfishness. The opportunity was there, so they took it.
People with Impulse Control Disorder (ICD) can find it impossible to resist temptation. ICD includes compulsive sexual behaviour (CSB) which usually starts at around 18 years of age (although it wouldn't surprise me if the age of onset occurs evermore earlier with such easy access to online pornography). 88.5% of sufferers are male who may also suffer from anxiety and/or depression .
Looking for more or different experience or knowledge. This can often be found with someone who's not a long-term partner.
12. Obsession with the other person
Being enchanted and entranced by someone who has perhaps become their best friend, their world and ultimately their lover. The person has fallen in love - and pursued it - with someone who is not their intimate partner or spouse.
13. A generally poor relationship
A relationship in which essential emotional needs aren't met in balance. It's never been right, they feel they’ve outgrown the relationship or have a general sense of dissatisfaction and disillusionment.
14. Narcissistic behaviour
This comes from a sense of entitlement. See my article: How to deal with a narcissistic spouse or partner.
15. Love of power
As human beings we have a need for a sense of control and volition. For some people, however, this essential emotional need has gone awry (see also # 4, 9 and 14).
16. Feeling like they've made a huge error of some kind - perhaps they've previously been unfaithful
They feel they've got nothing left to lose. They feel condemned anyway.
17. Sex, porn or erotica addiction
The need for sex trumps all, and what happens in the bedroom at home doesn't fulfil that powerful craving (see #10). See also: Signs of porn addiction.
18. Longing for an emotional connection
The partner is searching for intensity, and is longing to reconnect with a lost part of themselves.
Not understanding how precious and fragile an intimate couple relationship or marriage is and what the short and long term consequences are of betrayal (see: Surviving infidelity).
"There's intense longing to somehow feel alive again, and infidelity offers the desired excitement." Eli Finkel 
See also: Bored in your relationship?
So, why does someone cheat? I hope the list above has given you a better idea of what might cause someone to be unfaithful.
Don't forget - a reason is never an excuse for infidelity. But whether you're the wronged OR the unfaithful partner, understanding why it happened is key to fixing the fall-out.
Unless you know what and where the problems are you won't be able to address them. So if you're recovering from the revelation that your partner's had an affair, or you've been unfaithful and you never want to be so again, take some time to figure out where the cheating came from in your particular situation.
Together with reading the other articles on my site about infidelity, some careful thought can help you decide on the best way forward for you and your relationship right now. Cheating doesn't have to spell the end if you don't want it to...
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1. Sage Journals: Journal of Family Issues - Family Background and Propensity to Engage in Infidelity
2. Psychiatric Times: Impulse Control Disorders: Clinical Characteristics and Pharmacological Management
3. TEDxUChicago: The Marriage Hack, by Eli Finkel