From the Book, A Hunger For God by John Piper

Why I Wrote This Book

My aim and my prayer in writing this book is that it might awaken
a hunger for the supremacy of God in all things for the joy of all
peoples. Fasting proves the presence, and fans the flame, of that
hunger. It is an intensifier of spiritual desire. It is a faithful enemy
of fatal bondage to innocent things. It is the physical exclamation
point at the end of the sentence: “This much, O God, I long for
you and for the manifestation of your glory in the world!”

One might think that those who feast most often on communion
with God are least hungry. They turn often from the
innocent pleasures of the world to linger more directly in the presence
of God through the revelation of his Word. And there they
eat the Bread of Heaven and drink the Living Water by meditation
and faith. But, paradoxically, it is not so that they are the
least hungry saints. The opposite is the case. The strongest, most
mature Christians I have ever met are the hungriest for God. It
might seem that those who eat most would be least hungry. But
that’s not the way it works with an inexhaustible fountain, and
an infinite feast, and a glorious Lord.

When you take your stand on the finished work of God in
Christ, and begin to drink at the River of Life and eat the Bread
of Heaven, and know that you have found the end of all your
longings, you only get hungrier for God. The more satisfaction
you experience from God, while still in this world, the greater
your desire for the next. For, as C. S. Lewis said, “Our best havings
are wantings.”

The more deeply you walk with Christ, the hungrier you get
for Christ . . . the more homesick you get for heaven . . . the more
you want “all the fullness of God” . . . the more you want to be
done with sin . . . the more you want the Bridegroom to come
again . . . the more you want the Church revived and purified
with the beauty of Jesus . . . the more you want a great awakening
to God’s reality in the cities . . . the more you want to see the
light of the gospel of the glory of Christ penetrate the darkness
of all the unreached peoples of the world . . . the more you want
to see false worldviews yield to the force of Truth . . . the more
you want to see pain relieved and tears wiped away and death
destroyed . . . the more you long for every wrong to be made right
and the justice and grace of God to fill the earth like the waters
cover the sea.

If you don’t feel strong desires for the manifestation of the
glory of God, it is not because you have drunk deeply and are
satisfied. It is because you have nibbled so long at the table of
the world. Your soul is stuffed with small things, and there is
no room for the great. God did not create you for this. There
is an appetite for God. And it can be awakened. I invite you to
turn from the dulling effects of food and the dangers of idolatry,
and to say with some simple fast: “This much, O God, I
want you.”

http://cdn.desiringgod.org/pdf/books_hfg/hfg_all.pdf