Modern infidelity is different than traditional infidelity and sits on top of the romantic ideal that you find "the one" and that if you have everything that you need at home, you have no reason to go looking elsewhere. And if you have an affair, it's a symptom of a flawed relationship. If you don't apply the deficiency model to the relationship, then you apply it to the person. The person who strays is selfish, immature, addicted suffers from insecure attachment. And the person who doesn't stray is the committed partner: mature, stable, and non-selfish.