Fredrickson's unconventional ideas are important to think about at this time of year. With Valentine's Day around the corner, many Americans are facing a grim reality: They are love-starved. Rates of loneliness are on the rise as social supports are disintegrating. In 1985, when the General Social Survey polled Americans on the number of confidants they have in their lives, the most common response was three. In 2004, when the survey was given again, the most common response was zero.

According to the University of Chicago's John Cacioppo, an expert on loneliness, and his co-author William Patrick, "at any given time, roughly 20 percent of individuals—that would be 60 million people in the U.S. alone—feel sufficiently isolated for it to be a major source of unhappiness in their lives." For older Americans, that number is closer to 35 percent. At the same time, rates of depression have been on the rise. In his 2011 book Flourish, the psychologist Martin Seligman notes that according to some estimates, depression is 10 times more prevalent now than it was five decades ago. Depression affects about 10 percent of the American population, according to the Centers for Disease Control.

A global poll taken last Valentine's Day showed that most married people—or those with a significant other—list their romantic partner as the greatest source of happiness in their lives. According to the same poll, nearly half of all single people are looking for a romantic partner, saying that finding a special person to love would contribute greatly to their happiness.

But to Fredrickson, these numbers reveal a "worldwide collapse of imagination," as she writes in her book. "Thinking of love purely as romance or commitment that you share with one special person—as it appears most on earth do—surely limits the health and happiness you derive" from love.

"My conception of love," she tells me, "gives hope to people who are single or divorced or widowed this Valentine's Day to find smaller ways to experience love."

You have to physically be with the person to experience the micro-moment. For example, if you and your significant other are not physically together—if you are reading this at work alone in your office—then you two are not in love. You may feel connected or bonded to your partner—you may long to be in his company—but your body is completely loveless.

To understand why, it's important to see how love works biologically. Like all emotions, love has a biochemical and physiological component. But unlike some of the other positive emotions, like joy or happiness, love cannot be kindled individually—it only exists in the physical connection between two people. Specifically, there are three players in the biological love system—mirror neurons, oxytocin, and vagal tone. Each involves connection and each contributes to those micro-moment of positivity resonance that Fredrickson calls love.