UC Riverside psychology professor Sonja Lyubomirsky examines the psychology of love:
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. (To quote a line from the 2004 film “Before Sunset,” about two former lovers who chance to meet again after a decade, if passion did not fade, “we would end up doing nothing at all with our lives.” ) 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 compassionate 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.