What drives a person to cheat?

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Let me clarify first that this is not a post to justify cheating. I have posted a topic about Double standards in cheating discussing how the genders are used as an excuse for cheating. Anyway, just been thinking a lot about cheating as a subject and how common it is in relationships. It's as if people have an on-hand information about it, either they cheated, cheated on or their friends were cheated on or the ones who actually cheated. Now I ask, what really drives a person to cheat? I personally think that it takes 2 to tango in a relationship. One of the couple cheats because there's something lacking in his/her partner that he/she finds in another person. But then we're not perfect so there's always something lacking in a relationship or within that person but still some couples manage not to cheat. That's why I posted this topic to learn what really drives people to do it. Please post your opinions or your personal experiences. Thank you.