People cheat for a numerous reasons. In some instances it has nothing to do with their partner at all. Some people bore easily and need "variety". There is nothing their mate could have done to keep them from cheating. These are called "Incessant Cheaters". Most likely they have never had a long-term monogamous relationship in their lives. These types have charisma, flirt a lot, use sexual innuendo with strangers, and are generally adored or popular among crowds.
Another group of cheaters are the "Unbelievable Opportunity Cheaters." They don't necessarily proactively look to cheat. They cave into temptation if a "once in a lifetime" opportunity presents itself to be with someone they've had a secret crush on or fantasized about. Frequently this happens under perfect a scenario where they aren't likely to get caught such as a business trip. This type of cheater however often feels guilty afterwards if she or he has a loving partner and may eventually confess months or years later to absolve themselves of guilt feelings.
Last but not least is the "discontented cheater". He or she is very unhappy with some aspect of their marriage. Their partner may be neglecting them, taking them for granted, stopped being romantic, or verbally abusive. However they may be a good provider or do other things to enhance the couple's lifestyle. The goal of this cheater is to hold onto all that is good in their primary relationship while addressing their other needs on the "side". They feel (justified).