In fact, prairie voles are miserable when they are separated from their bonded partner for very long. A stress-related chemical called corticotropin releasing factor (CRF) acts in males separated from their female mates just the way it does in drug addicts who are separated from their supply. "Divorce" a male vole from his mate and you get a very miserable vole whose CRF system has been fired like a gun, triggering yearning and depression. The chemical is helping enforce social monogamy.
The neurochemical dopamine is motivational. It drives us to act to appease a desire, such as for food or sex, and when we do, we get a reward, typically a burst of endogenous opioids. With experience, we learn just how pleasurable it can be to tickle this reward system.
So our brains are organized according to chemically controlled circuits, each whispering to us about what it wants. When we see an attractive man or woman, reward circuits tell us how incredibly hot sex with that person would be. But oxytocin- and vasopressin-related circuits are telling us we love our partner, and CRF is helping us picture how miserable we'd be without our mate. The rational part of our brain, primarily the prefrontal cortex, is weighing these possible costs of cheating, and reminding us that the sexy person is married to our boss.
Which system shouts the loudest may depend partly on our genes. But one person's genome is not exactly like another's. We have variation. As we explain in our book, "The Chemistry Between Us: Love, Sex, and the Science of Attraction," this variation can make a lot of difference. When a European team studied monogamous birds called great tits, they found that 13 percent of chicks resulted from extra-pair mating. The birds, both male and female, most likely to fly off to find a paramour tended have "bold" personalities. This gregarious, novelty-seeking personality has been linked to a variation in a gene that holds the recipe for a dopamine receptor called D4, or DRD4 in humans.