Negative feedback
Negative feedback is a principle of anything sexual. It is equally present in the realm of sexual physiology and sexual psychology.
Any sexual stimulus loses its power the more often or the longer it is experienced or applied.
In the realm of psychology, everybody, sooner or later, gets bored with one and the same sexual partner all over again.
In the realm of endocrine physiology, any pro-sexual hormonal spike is followed by a hypothalamic impulse to down-regulate precisely this spike.
And in the realm of pharmacological libido enhancement, dopaminergic drugs noticeably decline in effectiveness after a number of applications.
It's our destiny to constantly be in search of the new thrill. And the more successful we are in colonizing new territory, the more we need to expand. You can't beat the system. We are only ever satisfied for the moment, but in reality, we never have enough