opponent-process theory of motivation

n. The hypothesis that when any stimulus arouses an affective response, it also arouses its opposite but to a lesser degree, thereby reducing the intensity of the original affect so that emotion is a combination of the two. The secondary affect is supposed to be aroused more slowly, to be less intense, but to endure longer than the original one so that after an emotion occurs it is likely to be replaced by its opposite but at a markedly reduced intensity. Thus after experiencing joy we experience a letdown which is a subdued form of sadness.