Let it suffice to say, then, that God perceives the imperfections within us, and because of his love for us, urges us to grow up. His love is not content to leave us in our weakness, and for this reason he takes us into a dark night. He weans us from all of the pleasures by giving us dry times and inward darkness.
In doing so he is able to take away all these vices and create virtues within us. Through the dark night pride becomes humility, greed becomes simplicity, wrath becomes contentment, luxury becomes peace, gluttony becomes moderation, envy becomes joy, and sloth becomes strength. No soul will ever grow deep in the spiritual life unless God works passively in that soul by means of the dark night.
ed. Richard J. Foster and James Bryan Smith