As it has been widely said - Men and Women are different. Concurred!! What makes us different is the way in which make our decisions - that is our thought process. MEN tend make their decisions from a LOGIC standpoint whereas WOMEN usually makes decisions based on EMOTION.
WOMEN

REVENGE - Most women tend cheat as of result of being hurt in the present relationships. Depending on whom they are dating or married to, it could be something they have enduring for quite some time and thus out of fatigue of the whole situation women will veer off into wrong doing herself.
LACK OF INTIMACY - At times relationships appear very happy and content on the surface, but on the deep end of it there are troubles. Sex is just one ingridient in the intimacy sector. A woman wants to feel wanted and loved. They want to be touched, hugged, spend quality time. Anything that will give her a sense of closeness. If these things are lacking, then expect certain things to change on her end and perhaps that just may be another man.
SELF - ESTEEM - If your girlfriend or wife has self esteem issues, cheating may satisfy her need to feel good about herself. However this does not mean she is inclined to take that kind of action, it all depends on how the guy beside her makes her feel. If he pays her full attention, pays her compliments - there may be nothing to worry about.
BEDROOM MONOTONY - Have you ever of the saying "Women Just wanna have fun", well its true even in the bedroom. They want you to be creative in every way possible in the bedroom. They can lose interest in sex just as quickly as they become interested in the first place. If it gets monotonous, then its downhill from there, she will lose interest in having sex with you, and eventually even lose interest in you.
NEGLECT - Women want to feel appreciated and loved. They want to hear certain things like "Thank you", "I love you", "I missed you", " I was just thinking of you". They never want to feel like you could care less althought at times it can wake them up and show you the same in return, because it can happen the other way around.
MEN
EMOTIONAL DISSATISFACTION - As much as tabloids and people seem to say about men not being emoitional beings. It is not all about the physical intimacy as society usually portrays men to be. Men have a need to feel appreciated as well, they are just not very good at expressing their emotions as women are. Although men don't explicitly ask for, they need affirmation as well.
LACK OF INTIMACY -Men also need her to be creative. They want a ladies in the STREETS and a freak in the bedroom. He wants to know that his girlfriend /wife wants be as intimate with him as he wants to be with her. One big turn off for men is a girlfriend / wife whom constantly turns him down for sex or other sexual adventure.
CONSTANT PETTY ARGUMENTS - There is nothing more annoying then a women who constantly nags and loves to argue. This tends to drive the relationship apart. Ok, men are not perfect and at times they need a push to do certain things, but if overdone it can be detrimental to the relationship, which causes them to lose interest and eventually begin to look at other ladies that may treat him better.
EGO BOOST - Yes!! man are guilty of wanting to stroke their egos and the thought of being able to get the next hot girl eventhough he may already be taken is satisfying ego wise. But it also all depends on the woman he is with. Does she make him feel good when they are together? Does she stroke his ego?