There are personal, interpersonal and societal contexts for infidelity.
In the midst of breaking a promise to one’s partner – and infidelity is just that — one is not reflecting much. Anger may be a driver, vaguely registered, since a recent fight unresolved seems like a good excuse to justify feelings of neglect or one’s right to trample upon the other’s trust. And you may well, again unwittingly, be following the precedent of a previous family role model. So now your personal loyalty to one’s extended family overtakes your commitment to putting loyalty to your partner first.
Beyond the personal act, one must consider the pay off in creating an interpersonal triangle: it is usually a stopgap to reduce the tension of hanging in an important conversation with one’s partner. People need to learn to keep the conversation about intense feelings for just a few more seconds in some cases. That requires courage.
Through the media, TV, Radio and Movies and much written literature, society actually encourages breaking rather than keeping the promise. How many happily married celebrities get the ink and the screen time? And with the rise of secular over religious institutions where do we get messages about maintaining dignity and marital durability?