What does it mean that the reward must satisfy a craving?

The reward works because it satisfies an underlying craving — identify the craving.

Example: You decide to build a daily exercise habit. You try rewarding yourself with a smoothie after each workout — it lasts a week. You try allowing yourself thirty minutes of guilt-free television — it lasts five days. You try buying new running shoes as a milestone reward — it motivates exactly one run. None of the rewards stick because none of them address the actual craving. When you finally ask yourself what you are really missing, the answer is not nutrition, entertainment, or gear. It is social connection. You have been working from home for a year and the isolation has been quietly compounding. You join a running group that meets three mornings a week, and exercise becomes effortless — not because running got easier, but because the reward now satisfies the craving that was actually driving the loop.

Try this: Select a habit you are currently building or attempting to build. Write down the reward you have been using (or assuming). Now run Duhigg's craving isolation protocol: the next three times the cue fires, try a different reward each time. After each alternative reward, wait fifteen minutes and write down whether the craving is gone or still present. If the craving persists, that reward missed the real craving. If it dissipates, you have found the underlying need. Document what you discover about the gap between the reward you assumed and the craving you actually have.