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.

It doesn’t take a scientist to observe that because the sex in a long-term committed monogamous relationship involves the same partner day after day after day, no one who is truly human (or mammalian) can maintain the same level of lust and ardor that he or she experienced when that love was uncharted and new.

Is the point that we should accept our biology or transcend it?

 that is the question of modern love, i do think.  the other factor is how different offspring turn out under each of the possible circumstances...ie half of all American kids are now raised by single moms...