The love heroin that bathes your brain in natural opiates when you are in love tapers off at about 4 years into a relationship. You are no longer the lucky recipient of a cocktail of blood-borne painkillers.
It is at this point that divorce becomes likely, particularly when the two erstwhile lovebirds don’t have anything else to bind them together: a strong friendship, commitment to a shared goal, commitment at the bone level to “in sickness and in health,” children… whatever works for you.
Basically, most people give up because it stops feeling good.
What a waste.