Why do we feel nostalgic?

Say you’re listening to the radio on your way to work, and a favorite song from 20 years ago starts playing. A sense of wistfulness overcomes you, evoking fond memories of the past. A familiar smell can elicit a similar sensation, as can a movie resurrected from childhood (Jurassic Park, anyone?). The feeling is bittersweet, though ultimately pleasant. Sound familiar? You’ve experienced nostalgia.