"I gather that the dopaminergic system in the reward centres of the brain respond even more vigorously to the expectation of reward than to reward itself. Hence, perhaps, the disappointment."

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Balance. It was all about balance. That had been one of the first things that she had learned: the centre of the seesaw has neither up nor down, but upness and downness flow through it while it remains unmoved. You had to be the centre of the seesaw so the pain flowed through you, not into you. It was very hard. But she could do it!

— Terry Pratchett