A few years ago, Brandon Green was sitting on the couch, brooding over a small mistake he'd made at work and the likely consequences. His roommate walked in and began telling him about something funny that happened to him that day. Mr. Green didn't chuckle or even smile. He scowled and said nothing.

That's when his roommate offered Mr. Green some valuable analysis: "Never mind," he said. "You're just not a happy person."

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