The emotions exist in pairs of opposites. Our swing into one emotion sets up readiness for a swing into the opposite emotion.
There is no up without down, no left without right, no forward without backward, all things tangible and intangible have two sides or two ends. Like complimentary or opposite colours that make possible each other, even produce their opposite colours in the eye. So too, emotions are the same. The existence of one emotion enables its opposite. And all are as if in a wheel, like the colour wheel.