God said, "When you seek me with all your heart, I will be found by you" (Jeremiah 29:13,14).

When a man or woman is willing to set aside the legitimate appetites of the body to concentrate on the work of praying, they are demonstrating that they mean business, that they are seeking God with all their heart.
Fasting is an expression of wholeheartedness. This is clear from Joel's call to the nation of Israel: "Yet even now," says the Lord, "return to me with all your heart, with fasting. . ." (Joel 2:12).
Andrew Murray said, "Fasting helps to express, to deepen, and to confirm the resolution that we are ready to sacrifice anything - to sacrifice ourselves - to attain what we seek for the kingdom of God."
How do you know when to pray and fast and when to just pray? That is not a question that someone else can always answer for you. But here is a principle:

In God's word we always find fasting connected with a very troubled spirit or a very anxious heart before the Lord. So a reason for fasting is not something you choose on the spur of the moment. Rather the reason is a consuming one. In a sense, it's not something you choose, so much as something that chooses you, because it's that important.
So why fast? To demonstrate that we are seeking God "with all our heart."

Fasting puts things in proper focus. It is a physical way of saying, "Food and the things of this life are not as important to me now as (fill in the blank)."
Of course, denying yourself food to focus on God and His program shows humility.

Let us humble ourselves, turn from our wicked ways, and seek God's face for our Nation and our world.