When I was a bummed-out teenager, I hit on an exercise that seemed magically to make me feel better. I called it the Happy List. I took out a few sheets of notebook paper and brainstormed as many things that made me happy as I could.

Sleeping in crisp white sheets.

Sitting in the sunshine.

Cuddling with my cat.

Watching the movie “Amadeus” for the umpteenth time.

I knew intuitively what researchers have since verified: Focusing on the positive and writing it down can lift one’s mood and impr...