Fasting challenges self-gratification. In fasting we are telling our bodies to temporarily give up one of our most basic needs in order to seek first the kingdom of God.
Confession: I am speaking purely theoretically. On a few occasions I have fasted a single meal, never more than that, and I have not done so recently. Indeed, this little study has convicted me of my own bent toward self-gratification. Perhaps I should put some of this into practice. Another word of warning: Since I haven’t incorporated this practice into my regular Christian life, and because I don’t know many people who have, I haven’t had to refine my thinking on the subject. The thoughts below are brief and preliminary and open to critique.
I first started thinking about fasting after reading Maslow’s A Theory of Human Motivation (see an introduction here). Essentially, Maslow postulates the so-called pyramid of needs, the fulfillment of which lead to health and happiness. These needs are fulfilled in order. Our most basic needs are physiological needs – hunger, thirst, etc. These needs are “pre-potent,” that is, “higher” needs (esteem, love, self-actualization) do not become conscious until we have gratified these basic needs. For a chronically hungry man “freedom, love, community feeling, respect, philosophy, may all be waved aside as fripperies which are useless since they fail to fill the stomach. Such a man may fairly be said to live on bread alone.” (emphasis mine) 
Immediately I thought of Jesus’ temptation in the wilderness (Matthew 4:1-11). “After fasting forty days and forty nights, he was hungry.” By far Jesus’ most pressing need was for food. By Maslow’s estimation, he should have had no other thought, no other desire, than to fill his stomach. At that moment, “The tempter came to him and said, ‘If you are the Son of God, tell these stones to become bread’” Jesus, being the Son of God, could have made it happen. Instead he responded:
“It is written: ‘Man does not live by bread alone, but on every word that comes from the mouth of God.’”
I don’t believe Jesus was invoking some superhuman ability at this point. He did not lay aside his humanity for the moment while he was being tempted. He was able to resist temptation because he relied on the Holy Spirit and, I believe, because His values were aligned with the values of the Father. He knew that his greatest need was not to fill his belly, but to be sustained by the very word of God and walk in obedience to the Father.
This brings us to what I think it is one of the most interesting paragraphs in A Theory of Human Motivation. In seeking to explain why his ordering doesn’t always appear to translate in the real world he states:
Perhaps more important than all these exceptions are the ones that involve ideals, high social standards, high values and the like. With such values people become martyrs; they give up everything for the sake of a particular ideal, or value. These people may be understood, at least in part, by reference to one basic concept (or hypothesis) which may be called ‘increased frustration-tolerance through early gratification’. People who have been satisfied in their basic needs throughout their lives, particularly in their earlier years, seem to develop exceptional power to withstand present or future thwarting of these needs simply because they have strong, healthy character structure as a result of basic satisfaction. They are the ‘strong’ people who can easily weather disagreement or opposition, who can swim against the stream of public opinion and who can stand up for the truth at great personal cost. It is just the ones who have loved and been well loved, and who have had many deep friendships who can hold out against hatred, rejection or persecution.
There are two essential points here for the Christian walk. First, commitment to high values is essential if we don’t want to be ruled by our base desires. This should not surprise us since central to obedience is the command to “love (value) the Lord your God with all your heart, mind, soul, and strength.” Re-ordered love/value is essential to spiritual formation. Jesus most certainly was a man of strong moral conviction. His ordered values allowed him to place obedience to God over filling his stomach in the midst of what must have been crushing hunger.
Second, in the words of Maslow, “it is just the ones who have loved and been well loved… who can hold out against hatred, rejection or persecution.” While Maslow saw this from a human perspective, I think the spiritual principle holds as well. Because Jesus loved the Father and was well loved by the Father, he was able to stand in the midst of suffering. Because He was sustained by “every word that proceeds from the mouth of God” he was able to forgo, for a time, filling his belly with bread.
There are many implications for the Christian walk (maybe to be expanded later). But I set out to apply this to the spiritual discipline of fasting so let me try to fulfill that goal.
First, in the Bible, fasting was often accompanied with prayer. It’s a time to practice Jesus’ command to “seek first the kingdom of God.” It is a time to consciously remind ourselves that communion with the Father, through the Son, in the power of the Spirit, is more essential to our lives than even our most basic physiological needs.
Second, in an age of consumerism (You need to fulfill your desires now! Buy this product!) fasting can help us add some much needed self-discipline and teach us to wait on God and to seek obedience to Him over quick self-gratification, a skill that can carry over into other areas of the Christian walk.
 Abraham Maslow, A Theory of Human Motivation.