Joy isn't the reward. It's the setup.
A psychologist noticed that positive emotions don't fit the model built for fear and anger, and the mismatch pointed to what they're actually for.
Fear narrows you, usefully. It deletes every option except run or freeze, which is exactly what you need mid-emergency. For decades, emotion research was built almost entirely on this template — an emotion's job is to trigger one sharp, specific action.
The problem the psychologist Barbara Fredrickson identified in the late 1990s was that joy, interest, and contentment don't behave that way at all. In her experiments, people who'd just watched a clip that made them feel joyful or content, when asked what they'd like to do right now, generated longer lists of options than people in a neutral state — while fear and anger shrank the list, exactly as expected. She called this broaden-and-build: positive emotions don't narrow toward one action, they widen the range of things that occur to you as worth trying.
The “build” half is the part that compounds. A single afternoon of curiosity doesn't feel like much, but multiplied across a life, it's how you accumulate skills, friendships, and options you'll draw on decades later. Joy isn't a reward sitting at the end of effort. It's the mechanism that makes the next effort occur to you at all.