American and European researchers tracked 1,761 people who got married and stayed married over the course of 15 years. The findings from this and other similar studies have been clear: newlyweds enjoy a big happiness boost that lasts, on average, for just two years…then they are back where they started…new love seems nearly as vulnerable to hedonic adaptation as a new job, a new home, a new coat and other novel sources of pleasure and well-being. (Though the thrill of a new material acquisition generally fades faster.)
Sexual passion and arousal are particularly prone to hedonic adaptation…both men and women are less aroused after they have repeatedly viewed the same erotic pictures or engaged in similar sexual fantasies. Familiarity may or may not breed contempt; but research suggests that it breeds indifference … There are evolutionary, physiological and practical reasons passionate love is unlikely to endure for long. If we obsessed, endlessly, about our partners and had sex with them multiple times a day — every day — we would not be very productive at work or attentive to our children, our friends or our health…Indeed, the condition of being in love has a lot in common with the state of addiction and narcissism; if unabated, it will eventually exact a toll.
WHY, then, is the natural shift from passionate to companionate love often such a letdown? Because, although we may not realize it, we are biologically hard-wired to crave variety. Variety and novelty affect the brain in much the same way that drugs do — that is, they trigger activity that involves the neurotransmitter dopamine, as do pharmacological highs...Evolutionary biologists believe that sexual variety is adaptive, and that it evolved to prevent incest and inbreeding in ancestral environments. The idea is that when our spouse becomes as familiar to us as a sibling — when we’ve become family — we cease to be sexually attracted to each other.