Emotions are contagious. "You can catch a mood, just like a cold," says Marco Iacoboni, M.D., author ofMirroring People: the Science of Empathy: How We Connect with Others. When you watch a person doing something, the mirror neurons in your brain are activated, even if you're not doing anything yourself. So if you see someone smile, your mirror neurons for smiling fire up, whether or not you're in a good mood. "Spend time with happy people and you'll eventually feel happier yourself," Dr. Iacoboni says.