Nature seems (the more we look into it) made up of antipathies: without something to hate, we should lose the very spring of thought and action. Life would turn to a stagnant pool, were it not ruffled by the jarring interests, the unruly passions of men. The white streak in our own fortunes is brightened (or just rendered visible) by making all round it as dark as possible, so the rainbow paints its form upon the cloud. Is it pride? Is it envy? Is it force of contrast? Is it weakness or malice? But so it is, that there is a secret affinity, a hankering after evil in the human mind, and that it takes a perverse, but fortunate delight in mischief, since it is a never-failing source of satisfaction. Pure good soon grows insipid, wants variety and spirit. Pain is a bitter-sweet which never surfeits. Love turns, with a little indulgence, to indifference and disgust: hatred alone is immortal.
We can interpret this passage as representing both Hazlitt’s underlying psychology of human behavior and his aesthetics. From the psychological standpoint, he seems to be saying that the happiness we seek is not arrived at through a cessation in tension but through the proper amount of stimulation, which must be endlessly recalibrated. We go through life like an electromagnetic needle nervously agitating between the undesirable poles of alpha-flat zombie and tortured suffering, trying to find the right voltage of bracingly vivifying pleasure/pain in the middle.