Some thoughts from John Piper's A Hunger for God, pp. 13-15:

"The birthplace of Christian fasting is homesickness for God."

"My hunger for food was silenced by the hunger of my heart."

"Half of Christian fasting is that our physical appetite is lost because our homesickness for God is so intense. The other half is that our homesickness for God is threatened because our physical appetites are so intense. In the first half, appetite is lost. In the second half, appetite is resisted. In the first, we yield to the higher hunger that is. In the second, we fight for the higher hunger that isn't. Christian fasting is not only the spontaneous effect of a superior satisfaction in God; it is also a chosen weapon against every force in the world that would take that satisfaction away."

"The greatest enemy of hunger for God is not poison but apple pie. It is not the banquet of the wicked that dulls our appetite for heaven, but endless nibbling at the table of the world. It is not the X-rated video, but the prime time dribble of triviality we drink in every night."

"The greatest adversary of love to God is not his enemies but his gifts. And the most deadly appetites are not for the poison of evil, but for the simple pleasures of earth. For when these replace an appetite for God himself, the idolatry is scarcely recognizable, and almost incurable."