A song you have not heard in years can change your mood in seconds. One chorus, and suddenly you are back in a car with your family, in a school hallway, or in a bedroom covered with posters from another version of your life. Nostalgia works that fast. It is not just remembering. It is feeling the past in the present.Nostalgia is one of the most familiar human emotions, yet it is often misunderstood. People talk about it as if it were simple homesickness or a habit of living in the past. In real...