Sex therapist Dr. Laura Berman says attraction has a lot more to do with science and evolution than people might think. "We are innately all puppies in heat," she says. "We are capable of discerning 10,000 different scents consciously. But then there's a whole realm of unconscious scents that we're not even aware that we're smelling."

These odors let people know when a woman is fertile and when a man's testosterone levels are high, Dr. Berman says. As a result, individuals can tell when someone else is most ripe for reproduction. "It's all about survival of the species," she says.