Extra-marital affairs are responsible for the 20-40% breakdown of most marriages and end in divorce. This is one of the most common causes of divorce. The reasons why people cheat aren’t as cut and dry as our anger may lead us to believe. Anger and resentment are common underlying reasons for cheating, along with differences in ....appetite and lack of emotional intimacy. Infidelity often begins as a seemingly innocent friendship, says cheating expert Ruth Houston. “It starts as an emotional affair which later becomes a physical affair.” Infidelity is one of the primary reasons for divorce. It is also one of the legal divorce reasons, besides living apart for more than a year and subjecting your partner to cruelty (mental or physical).