Rather, it is what she calls a "micro-moment of positivity resonance." She means that love is a connection, characterized by a flood of positive emotions, which you share with another person—any other person—whom you happen to connect with in the course of your day. You can experience these micro-moments with your romantic partner, child, or close friend. But you can also fall in love, however momentarily, with less likely candidates, like a stranger on the street, a colleague at work, or an attendant at a grocery store. Louis Armstrong put it best in "It's a Wonderful World" when he sang, "I see friends shaking hands, sayin 'how do you do?' / They're really sayin', 'I love you.'"

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. 

When you experience love, your brain mirrors the person's you are connecting with in a special way. Pioneering research by Princeton University's Uri Hasson shows what happens inside the brains of two people who connect in conversation. Because brains are scanned inside of noisy fMRI machines, where carrying on a conversation is nearly impossible, Hasson's team had his subjects mimic a natural conversation in an ingenious way. They recorded a young woman telling a lively, long, and circuitous story about her high school prom. Then, they played the recording for the participants in the study, who were listening to it as their brains were being scanned. Next, the researchers asked each participant to recreate the story so they, the researchers, could determine who was listening well and who was not. Good listeners, the logic goes, would probably be the ones who clicked in a natural conversation with the story-teller. 

Describing love as a "micro-moment of positivity resonance" seems like a buzz-killWhat they found was remarkable. In some cases, the brain patterns of the listener mirrored those of the storyteller after a short time gap. The listener needed time to process the story after all. In other cases, the brain activity was almost perfectly synchronized; there was no time lag at all between the speaker and the listener. But in some rare cases, if the listener was particularly tuned in to the story—if he was hanging on to every word of the story and really got it—his brain activity actually anticipated the story-teller's in some cortical areas. 

The mutual understanding and shared emotions, especially in that third category of listener, generated a micro-moment of love, which "is a single act, performed by two brains," as Fredrickson writes in her book.

Oxytocin, the so-called love and cuddle hormone, facilitates these moments of shared intimacy and is part of the mammalian "calm-and-connect" system (as opposed to the more stressful "fight-or-flight" system that closes us off to others). The hormone, which is released in huge quantities during sex, and in lesser amounts during other moments of intimate connection, works by making people feel more trusting and open to connection. This is the hormone of attachment and bonding that spikes during micro-moments of love. Researchers have found, for instance, that when a parent acts affectionately with his or her infant—through micro-moments of love like making eye contact, smiling, hugging, and playing—oxytocin levels in both the parent and the child rise in sync.