All sorts of reasons: Boredom probably being the "soil" for all the other reasons, such as just doing so, to see if one can get away with it. Good old fashioned lack of discipline could be yet another reason. Finally stupidity can not be ruled out either.
something is missing in the relationship. usually one the two won't discuss it, leaving the other one feeling lost. mainly not communicating needs and wants is key. if you don't, there will be anger and resentment. they look to someone else who understands
Because a situation presents itself or a person creates a situation where they get lucky. Their is no thought of how it may affect their partner or not enough that it prevents them doing it. If they do it often enough behind their partners back they evenuatally do it without guilt which shows what sort of character they possess. Self gratification at someone else's expense is no trait to brag about but those that do it & feel no remorse are arogant & until the shoe is on the other foot will never quite understand just how it hurts a partner whether they are going through a rough patch at the time or not. It's not right but it is part of human nature to want more than what they already have. The grass isnt always greener on the other side so you shouldn't mow someone else's grass.
cause they know they can get away it with... most women that let their men cheat and take them back... have no self respect and no self esteem... so they keep doing it.. cause they know you aren't going anywhere.... Stop the cycle.. stick up for youself and LEAVE
There are a lot of reasons that people cheat or are unfaithful. Certainly the level of maturity and commitment to a relationship influences those "decisions". With social media it is so easy to find other interesting partners and this often leads to cheating. For fuller answers to this question, check out the article "Affairs and Causes. http://counselingrelationshipsonline.com/Causes-of-an-Affair.html