The Will to Power · Friedrich Wilhelm Nietzsche
702.
Man does not seek happiness and does not avoid unhappiness. Everybody knows the famous prejudices I here contradict. Pleasure and pain are mere results, mere accompanying phenomena--that which every man, which every tiny particle of a living organism will have, is an increase of power. In striving after this, pleasure and pain are encountered; it is owing to that will that the organism seeks opposition and requires that which stands in its way.... Pain as the hindrance of its will to power is therefore a normal feature, a natural ingredient of every organic phenomenon; man does not avoid it, on the contrary, he is constantly in need of it: every triumph, every feeling of pleasure, every event presupposes an obstacle overcome. Let us take the simplest case, that of primitive nourishment; the protoplasm extends its pseudopodia in order to seek for that which resists it,--it does not do so out of hunger, but owing to its will to power. Then it makes the attempt to overcome, to appropriate, and to incorporate that with which it comes into contact--what people call "nourishment" is merely a derivative, a utilitarian application, of the primordial will to become stronger. Pain is so far from acting as a diminution of our feeling of power, that it actually forms in the majority of cases a spur to this feeling,--the obstacle is the stimulus of the will to power.
Text from Project Gutenberg, public domain in the United States.