Many people who consider themselves to be a victim of infidelity are looking for a reason or an explanation for why their partner pursued a physical relationship with another person. There is one simple explanation for this that is applicable to anyone reading this article and hoping to learn why infidelity occurs: certain needs were no longer being met.
On a fundamental basis, an intimate relationship is no different than a business relationship. Apple wanted a lot of hype to surround the release of their iPhone. AT&T wanted to provide consumers with a good reason to choose their service over a competitor's. The two companies realized that the whole would be greater than the sum of the parts if they made an agreement to release the iPhone exclusively to AT&T customers. Intimate relationships work in exactly this fashion. One person has a certain set of needs. They meet another person with a set of needs of their own. The two people realize that they might be able to provide for each other's needs while simultaneously having their own needs provided for. In that moment, their whole is greater than the sum of their parts, and so a relationship begins.
The most successful relationships are the ones where the people are able to adapt to each other's changing needs. People's needs are always changing. Eventually they disappear altogether. This is why the great spiritual leaders in our world have never been in a relationship. It wasn't a need of theirs. They have no (or at least very few) needs. They simply give.
When a person enters a relationship, one of three things will happen: a.) through the therapy provided by their significant other, their needs will be met so abundantly that they will eventually want for nothing, b.) the person will continue having needs that are constantly evolving in their depth and characteristics, but their mate will be aware of this and continue to provide them with their needs, at least enough to ensure that their own need of monogamy is met, or c.) the person's needs will eventually no longer be met by their spouse.
When the last of these three possibilities occur, quite understandably, the person will look elsewhere for their needs. Sometimes this results in what many refer to as "infidelity".
When a person engages in an act of intimacy with another person, it could seem intuitive to think that they had a physical need that was no longer being met. But this is not always the case. It could be that they had emotional needs that were not being met which had no direct relation to sexuality. These needs could be as simple as sharing the details of their life with someone. An act of intimacy can occur if the person they find to fill this void in their lives has a need of their own they want in exchange: sex.
One of many huge fallacies of our society is that relationships whose whole is greater than the sum of the parts will always remain that way. This fallacy has millions of people each year entering into an institution we call "marriage", which states that they will be together forever. It would be unreasonable, perhaps even downright unfair to expect another person to constantly, always, and forever be prepared to provide for your needs. Even more, it would be unfair to expect one's self to be able to do this for another.
After reading that last paragraph, you might be thinking to yourself, "well you make it sound like infidelity is inevitable if we place expectations [like staying together forever] on relationships." If you are thinking that, you are absolutely correct.