Because this person is emotionally weak. They have poor coping skills. Their world view stems from selfishness, immaturity and a sense of entitlement. Given certain stressors, lack of happiness, boredom, not enough sex, feeling misunderstood, etc. they feel they deserve to have those happy love feelings.
While this might be true for a lot of people that cheat, it certainly can't be used as a blanket statement for all people that cheat. There are others out there that cheat but aren't selfish, immature or have a sense of entitlement. I know that everyone on here wants to think that the LBS is great and the WAS are horrible selfish jerks that just should have stayed put, but that's not true either. Now I don't condone cheating, but sometimes people try other things and can't find a way out because of the kids, or money, abusive spouse, or whatever. And, sometimes the LBS ignore all attempts to fix the marriage because of their own problems.
I'm not saying that this has anything at all to do with you Tyler. You seem like a nice guy. I'm just saying in general, that your W may have reasons other than what was stated above for her thought process that led her down the wrong path.
M:45 H:48 M:11 No kids BD:Sept'15 EA:Confirmed 1wk later PA: Oct'15 12 '15 2 wk R Just kidding, H wants NC 12 '15 H back w/OW 4 '15 R &still working on it