Most of this entry is lifted from an op-ed piece in today’s New York Times; preparing to sing in eight Christmas concerts over the next few weeks is taking up so much of my time that I had to resort to a less-than-original posting today.

When love is new, we have the rare capacity to experience great happiness while being stuck in traffic or getting our teeth cleaned. We are in the throes of what researchers call passionate love (or, to use the technical term, limerence), a state of intense longing, desire and attraction. In time, this love generally morphs into companionate love, a less impassioned blend of deep affection and connection. The reason is that human beings are, as more than a hundred studies show, prone to hedonic adaptation, a measurable and innate capacity to become habituated or inured to most life changes.

We’re inclined — psychologically and physiologically — to take positive experiences for granted. We move into a beautiful loft. Marry a wonderful partner. Earn our way to the top of our profession. How thrilling! For a time. Then, as if propelled by autonomic forces, our expectations change, multiply or expand and, as they do, we begin to take the new, improved circumstances for granted. Sexual passion and arousal are particularly prone to hedonic adaptation. It seems that 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.

WHY, then, is the natural shift from passionate to companionate love often such a letdown? For two reasons: first, we are culturally trained to expect lifelong pleasure and contentment in long-term relationships: don’t all the fairy tales promise “happily ever after?” Second, 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.

When partnered couples reach the two-year mark, many mistake the natural shift from passionate love to companionate love for incompatibility and unhappiness. For many, the possibility that things might be different — more exciting, more satisfying — with someone else proves difficult to resist. Injecting… surprise into even the most stable, seasoned relationship is a good hedge against such temptation. Surprise is a potent force. When something novel occurs, we tend to pay attention, to appreciate the experience or circumstance, and to remember it. We are less likely to take our marriage for granted when it continues to deliver strong emotional reactions in us.

The realization that your relationship no longer supplies the charge it formerly did is an invitation: shake up predictability in favor of discovery, novelty and opportunities for unpredictable pleasure. There are obvious (but often expensive) high-impact surprises that can be planned: a long weekend getaway to the Caribbean, a surprise birthday party, a new car. But even more important is a series of smaller surprises: preparing the dinner your partner would normally cook, a bouquet of flowers, a card with a romantic message written in it, a “gift certificate” for a backrub, a phone call in the middle of a workday to deliver an “I love you” message. And, of course, it’s important to think about surprises in the bedroom (although too much surprise won’t necessarily be appreciated, and may in fact be a turn-off. For many couples alluring attire, a new position, a reversal of the normal sequence or roles, or a toy.

“A relationship,” Woody Allen proclaimed in his film “Annie Hall,” “is like a shark. It has to constantly move forward or it dies.”