Comment by NickNaraghi

Csikszentmihalyi's flow research[0] basically predicts the author's whole arc here. People are happiest during structured, challenging activities with clear goals and tight feedback loops. Coaching middle schoolers in a gym hits every condition on his list.

Btw, the other finding worth mentioning is that people consistently predict that free time and relaxation will make them happier, then report the opposite.

[0] Flow: The Psychology of Optimal Experience if you haven't read it