Fasting is not a no to the goodness of food or the generosity of God in providing it. Rather, it is a way of saying, from time to time, that having more of the Giver surpasses having the gift. If a husband and wife resolve to give up sexual relations for a season to deal earnestly with a problem keeping them at odds, this is not a condemnation of sex but an exaltation of love. Food is good. But God is better. Normally we meet God in his good gifts and turn every enjoyment into worship with thanksgiving. But from time to time we need to test ourselves to see if we have begun to love his gifts in the place of God. –John Piper, A Hunger for God
This is one of the most important reasons to fast. For creatures who habitually breed contempt from familiar tokens of love, it is essential to regularly run diagnostics. Paul says that some people turn their stomachs into their god. Every right pleasure ought to indicate to it’s recipient the love and communion the creator of matter and sensation intended for his people. And there is nothing like hunger and thirst to bring to mind what our desires for God ought to be like and to confirm our utter dependence on his goodness and providence. In this sense, fasting is preparation for feasting just like not snacking all day sets one up for a hearty dinner.