I suppose that words, timely and arranged in the right order, produce an afterglow. When you read words like that in a book, beautiful words, a powerful but fleeting emotion ensues. And you also know that soon, it'll all be gone: the concept you just grasped and the emotion it produced. Then comes a need to possess that strange, ephemeral afterglow, and to hold on to that emotion. So you reread, underline, and perhaps even memorize and transcribe the words somewhere - in a notebook, on a napkin, on your hand.