From Piper's A Hunger for God, pp. 15-19:

"We [must] do anything and go without anything if, by any means, we might protect ourselves from the deadening effects of innocent delights and preserve the sweet longings of our homesickness for God. Not just food, but anything."

Piper paraphrasing Augustine: "We easily deceive ourselves that we love God unless our love is frequently put to the test, and we must show our preferences not merely with words but with sacrifice."

"Many small acts of preferring fellowship with God above food can form a habit of communion and contentment that makes one ready for the ultimate sacrifice. this is one way that fasting serves all our acts of love to God. It keeps the preferring faculty on alert and sharp. It does not let the issue rest. It forces us to ask repeatedly: do I hunger for God? Do I miss him? Do I long for him? Or have I begun to be content with his gifts?"