"Love" and loyalty and a happy long successful relationship aren't spiritual, they aren't written in the stars, they aren't an actual physical thing.

They don't "exist"

Attraction, enough to keep a happy long term relationship together, doesn't exist and is impossible to maintain....EVENTUALLY after the honeymoon period, things grind down whether it be flaws, incompatibility, the man beta'ing out or the opposite, being too uninvested, or plain good ole familiarity and too much comfort render the woman unattracted and bored.

BUT...people can choose to maintain a relationship. You can actively CHOOSE to rise above emotions and temporary flailings in attraction and ego's and instincts and all those things that whilst at the start of a relationship worked in their favour, as it drags on works to their detriment.

A couple can choose and mentally commit to staying loyal. It WILL be hard, monogamy is. Love isnt natural and it will never feel effortless. To make something long term work you both have to conciously commit to the challenge of working against your own attractio and disgust triggers and just push through, but this requires a self awareness and rationality most can't bring themselves to muster.

Essentially, a couple has to both agree to between them that they are going to "pretend" love, the spiritual fantasy version we all gobble up as naive children, is real, and to act as both actors but more importantly stage hands in maintaining that illusion for themselves.