This isn’t about the valleys. They’re bad enough they need no explanation.
It’s the damned middle; the non-peaks. Coming off a peak, the relative dullness of a given situation is magnified by the non-peakedness of the moment. Not only does one feel off because of the relative dullness, one magnifies the intensity of the displeasure by resisting the descent from the peak. IE, I feel droopy, I feel shitty about feeling droopy, and I feel shitty about not being on the peak. Triple whammy.