While the desire to cheat is a fundamental, and unconscious, part of our human nature, not everyone will be unfaithful.
Like most of our behaviors, infidelity is not intentional, but, for the most part, it is situationally driven.
When placed in the right situation (or wrong situation, in this case), our emotions can get can prompt us to act in ways which are counter to our beliefs. Please note that some of the factors listed below are drawn from Buss and Shackelford's work on infidelity.
What factors influence an individual's willingness to cheat?
All things being equal, an individual’s attractiveness influences how likely he or she is to cheat. Attraction comes in many different forms – it is influenced by one’s physical appearance, one’s social skills, and one’s tangible resources (money). The more one is in demand, the more likely one is to cheat. People, who have higher incomes, more education, and successful careers, are more likely to cheat than people who are less successful. And physical attractiveness also plays an important role.
Again, all things being equal, the more individual free time people have the more likely they are to cheat. Couples who have separate social lives, friends, careers, travel plans, and so on are much more likely to cheat than couples who spend most of their time together. The more opportunity people have to cheat, the greater the odds that cheating will occur.
People who like to take risks or have a sense of adventure are more likely to cheat than people who are more fearful or timid by nature. And there is most likely a genetic component involved in risk-taking behavior - some people may be predisposed to taking risks.
Sexual desire varies from person to person. Some people have a very high sex drive while other people are much less concerned or interested in sex. And people with a high, rather than low, sex drive are more likely to cheat. Again, sexual desire appears to be influenced by genetic factors. Some people are inherently more easily aroused and driven by their desire for sex than other people (see, webMD). People who have multiple affairs are often addicted to the novelty and excitement which infidelity can provide.
Attitude Toward Love and Romance
Attachment and Love Styles
Some people view love and romance as a sacred bond between two individuals. Other people see love as a game, where the goal is to manipulate another individual and gain emotional power over a partner. People who view love as a game are much more likely to have multiple love interests; cheating is just another way to gain control over one's spouse. Also, people with a dismissing style of attachment, are more likely to cheat.
As problems emerge in a relationship, people are more likely to cheat. Infidelity is more common in relationships where people feel misunderstood, under appreciated and where fighting and bickering is common.
Sense of Entitlement
Some people, due to their position in society, their beliefs about gender roles, or their cultural upbringing, believe that it is their right to cheat on their partners. In other words, some people believe that cheating is a privilege to which they are entitled. Such individuals, philanders, often engage in infidelity with little guilt or remorse.
Source: Truth About Deception