1. Opportunity, environmental conditions
That's because there's a chance. It's wide open. Besides, all are doing it. Whether you want it or not, it's conditioned for cheating so that you become normal among your friends, your neighborhood.
Due not to have the affair, it makes you as if you're a magical creature, and you don't wanna be labeled the goody-goody, the partner fear, and others. So, you deceive. You cheat because the current keeps you as if it'd be.
2. Accidentally fall in love
You swore, you didn't wanna fall in love again. During that time, you thought up to be sure that your partner was your true love, and it'd be forever and ever. Then, suddenly, you met someone. Whether in the immediate time or because you often met one another, after a long time, you fell in love more deep, more intensely, inevitably, and more and more crazy.
Then, you're not able to control the urge to realize your love to someone in behavior which refers to all forms of infidelity.
What's the point? It's an upbringing in a family very permissive of the affair that having romance relationships outside marriage is a very common thing. Loyalty isn't a big deal in the family. The value of commitment in the family is minimum, or even artificial.
Thus, all the things are brought along to adulthood, including when entering into the married life.
A payback, revenge for what? Revenge against everything that ever happened in your past, the trauma. What trauma? People have their own experiences. Because of that, it's highly subjective and casuistic. The answer to why isn't uniform, not due to a similar cause.
For example, it could be because at some point in your past, you were betrayed by your partner, your spouse so that later, you do the same thing.
Evenly, it might be, you yourself didn't experience it but others, such as your parents, your sibling, or your friend. Then, you take the traumatic experience as a reason to hold a grudge.
It could be a form of retaliation for the abuses you've endured, either you do seriously experience, or actually, it's only in your own mind. For example, you're ugly, poor, short, strange, and a variety of other reasons that you're not valuable enough for your partner.
In fact, it could be, you have an affair as a form of retaliation against your own fears, your own thoughts against an ailing relationship.
5. Need to fill void
The nature is indeed existential. Basically, people need to exist so that their presences in this life are meaningful. Nevertheless, basically, there're empty spaces waiting to fill in everyone, some voids. Occupancy of the vacancy is a yearning in yourself whether you realize or not.
Consciously or not, you attempt to fill the empty space. You then perform searches, and when you meet someone, you feel compatibility that arises from the chemistry that occurs when there's a relation. Chemistry happens on an exquisite taste sensation, such as happiness, fitness, love, and so forth, and you feel, he or she is your soulmate, your spouse.
The problem is whether one person is enough to meet all the empty spaces that you have. The answer is of course not. Therefore, after some time later, when the honeymoon period of relationships is over, again, you feel the empty spaces remain and haven't been fulfilled. Then, the search process begins again until you meet another person you feel fit.
Is that correct? Actually, it could be a yes or no. When you let go your spouse for the sake of him or her, automatically, will all your desires be fulfilled? It's obviously not. Why? He or she is able to complete the void because another empty space are covered by your own true mate.
Unfortunately, this is often not recognized by the perpetrator of infidelity.
You may also like:
The Cheating Culture
When Good People Have Affairs
The (Honest) Truth about Dishonesty
The Science of Good and Evil