Most Helpful Girl
Every person is different. I would guess that many of the reasons men cheat would apply to women too.
Usually people look outside a relationship for things they feel they aren't getting and/or can't ask for inside their relationship. This could be physical intimacy, emotional support, attention or excitement.
Some people are so scared to be alone that they cling to their falling relationship while trying to a supplement it externally. Another portion of people self sabotage. When things move past the excitement of falling in love they unconsciously sabotage the relationship (getting drunk and cheating, picking stupid fights, being unwilling to compromise...). They might make affairs more and more obvious so they will eventually be caught, forcing their partner to become the decision maker.2THIS IS NOT RELEVANT ANYMORE
Most Helpful Guy
Most girls cheat because they probably were not getting the amount of attention they would have ideally wanted in a relationship. Girl need a s**t ton of attention and compliments in a relationship to keep them happy.
A lot of guys on the other hand cheat due to weakness on seeing another girl who is willing to satisfy certain sexual wants their partners are not willing to.1THIS IS NOT RELEVANT ANYMORE