Is feeling this way about your spouse necessary for a good marriage? I asked.

“Absolutely not,” Dr. Helliwell said. “The benefits of marriage are strong even for those who are littered with outside friends. It’s just bigger for those who consider their spouse their closest friend. It’s a bonus.”

Others are not so sure.

Amir Levine is a psychiatrist and neuroscientist at Columbia University, and the co-author of “Attached.” A student of social relations, Dr. Levine explained that everyone has what he calls a hierarchy of attachment, meaning if something bad happens to us, we have a ranking of the people we call. In our early decades, those on the highest rungs are usually our parents or other family members.

“The problem as you grow older is, how do you let somebody close who’s basically a total stranger?” he said. “Nature came up with a trick: It’s called attraction. Sexual attraction brings down all the barriers, lets you get close to a new person in a physical way that you don’t get close to your family.”

Over time, of course, this physical connection wanes. While many bemoan this loss of titillation, Dr. Levine celebrates it. “It’s smart,” he said. “If you’re going to be crazy about the other person all the time, how are you going to raise kids? How are you going to be able to work?”

Instead of complaining, we should view this new phase as an achievement: “O.K., now I have this person I’m attached to. I have the feeling of security. That’s what allows me to be an individual again and self-actualize.”