Matthew 6:16-18 When you fast, do not look somber as the hypocrites do, for they disfigure their faces to show men they are fasting. I tell you the truth, they have received their reward in full. But when you fast, put oil on your head and wash your face, so that it will not be obvious to men that you are fasting, but only to your Father, who is unseen; and your Father , who sees what is done in secret, will reward you.
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 tham 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 place of God.
This is the essence of Christian fasting: We ache and yearn – and fast – to know more and more of all that God is for us in Jesus. But only because he has already laid hold of us and is drawing us ever forward and upward into “all the fullness of God.”
A Hunger For God, By John Piper, pg. 43, 48.