While Paul and I are phasing out of the excitement of new love, Sona Mody, 26, and Carl Bartsch, 29, are just getting started. The couple met at the birthday party of a mutual friend, and have been inseparable ever since. As a medical student, Mody doesn't have much free time, and wasn't looking for a relationship. But her connection with Bartsch was too strong to ignore. After three months together, Mody says, "I feel overwhelmed by how much I care about him. We said we loved each other a month in." Though Mody has had serious relationships in the past, she says this one is different. "It's a new feeling, a satisfying feeling. You go to bed content every night. It’s really inspiring."

Falling for each other, as it turns out, is just as much physical as it is emotional. New lovers "get a real surge of dopamine in their brains—in the same area that feels the effects of cocaine and that makes us reach for chocolate," Fisher says. They may also experience a rush of norepinephrine, the chemical that causes the heart to pound and the palms to sweat, she adds. "Infatuation is a state created by nature to get us to meet, mate, and procreate," says Pat Love, a relationship therapist and author of The Truth About Love: the Highs, the Lows and How You Can Make it Last Forever. "It's an altered state. Your brain wants you to pay attention so it gives you pleasure. You’re just flooded with dopamine."

Once in love, our hearts and brains are working overtime. But what draws us to a person in the first place? Abigail Goodman*, a single magazine editor in New York City, has noticed a strong shift in what attracts her to a certain man. Throughout her twenties she bounced from short relationship to short relationship, always pulled towards a specific type: tall and dark, with a big personality. "I used to be attracted to the guy who could hold the room, tell the jokes, command the dinner table," she says. Now 33, Goodman is no longer dating casually, and hopes to find someone with whom she can settle down and start a family. And she understands that he may not be the one she originally imagined. "These days I'm much more drawn to the quiet observer type," she says. "It took dating a couple of those [extroverted] people to realize that they’re usually making up for something else."

According to Fisher, like attracts like. "People tend to fall in love with someone of the same ethnicity, age, socioeconomic background, intellectual ability, level of attractiveness, and religious values," she explains. But subtle factors also affect who we choose as our mate. "I think we are unconsciously attracted to people who are chemically unlike us," she explains. "For example, a person who is high in dopamine is also curious, creative, and outgoing. That person is more likely to fall for someone who is high in testosterone, who is more likely to be conscientious, a scheduler, and a planner. This creates more variety in babies and brings a greater array of parenting skills to the family."