From Francis Chan and David Platt's introduction to the new edition of Piper's ‘A Hunger for God ’ :

If we really want to be a part of seeing disciples made and churches multiplied throughout North America and to the ends of the earth, we would be wise to begin on our knees....There is spiritual delight to be found in God that far supersedes the physical diet of this world, and fasting is the means by which we say to God, “More than our stomachs want food, our souls want you.” Once we “taste and see that the Lord is good” (Psalm 34:8), the things of the world no longer appeal to us in the same way....We fast because we’re hungry for God’s Word and God’s Spirit in our lives. We fast because we long for God’s glory to resound in the church and God’s praise to resound among the nations. We fast because we yearn for God’s Son to return and God’s kingdom to come. Ultimately we fast simply because we want God more than we want anything this world has to offer us.