Infidelity is one of the leading causes for divorce. Though cheating is often thought to be a conscious decision made by a partner, there are a variety of reasons infidelity occurs. It is more than an emotional detachment from your partner.
The types of relationships you have with people are predetermined in your genes. If you have a variation of the vasopressin receptor, it is more likely you cannot be intimate with a person. This leads to infidelity.
Mania, related to bipolar depression, is another cause of infidelity. When a person makes an impulsive decision without considering the consequences, it may have been caused by a manic outbreak.
Another cause of infidelity is the relationship itself. If there is more negative behavior between partners than positive, infidelity is more likely to occur.