But couples can lasso this predictability to keep the fire hot. They can take up activities that drive up dopamine -- a neurotransmitter associated with novelty and working for reward -- in such critical brain regions as the VTA. Kicking up that brain activity, in turn, increases feelings of romantic love.
"Any kind of novelty, any activity that's new, exciting, challenging, possibly dangerous, will work," said Helen Fisher, a biological anthropologist at Rutgers University and a co-author of the study. Ride through New York City in a pedicab after dark, go night sailing, go nude swimming, study a new kind of music, take a vacation, go out to the airport or simply open a map book and run your finger down a page and choose a country.
"It doesn't have to be that elaborate," said Fisher.
"Any kind of sexual stimulation is good," she said. "Don't wait to feel sexy. Just get into bed with your partner."
And, yes, the study's long-in-love marrieds said they had plenty of sex, or at least enough to set off a glow in the posterior hippocampus.
Fisher believes the study has implications for people in all stages of love, not just the long-in-love couples.
"Brain circuitry can be triggered at any time," said Fisher, who's the scientific adviser to Chemistry.com, a division of Match.com. "A lot of people head into courtship looking for fireworks. Don't pass up a chance by dumping someone after a first date because you don't feel the fireworks. The fireworks can happen at any time and be maintained."
Despite the similarities in brain activity of the long-in-love and the newly-in-love, the study found some telling, although not surprising differences.
"In that early-love stage, you're in that state of exhilaration," said Fisher. "You talk till dawn. You become obsessed with 'What does he think?' 'Does he like me?' 'Does he think I'm fat?'"
Susan Heitler, a Denver clinical psychologist and author of "The Power of Two: Secrets to a Strong and Loving Marriage," explained further. "In a funny way, high, initial romantic love is associated with the almost negative feeling of anxiety, whereas you get the positive high without the anxiety with the long-term love."
The scans of the long-in-love also showed activity in brain regions associated with attachment, liking and bonding. "It's the same area of the brain that lights up when there's a positive attachment between a parent and a child," said Heitler. "That's not to say it's infantilizing. It's attachment connectedness and liking."
Again, no action there for the newly-in-love folks. "They're not high in liking, and they're not high in attachment," said Heitler, who was not involved in the Stony Brook study.
Let's see: It seems that long-in-love marrieds can have the same intense, romantically tingly, sexy relationship as the besotted newly-in-love -- but without the anxiety, obsessive frenzy and disconnectedness.
"You can have both, minus the mixed-bag feeling of intense wanting with an obsessive quality. That's the negative side of romance, which can kind of take over your life," said Heitler. Perhaps it's time for all the long-in-love to uncork another bottle of Champagne and toast themselves -- again -- on this Valentine's Day. They should at least break open the chocolates.