The central idea in fasting is to voluntarily deny otherwise normal functions for the sake of intense spiritual activity. The life of the spirit impregnates, infiltrates, and dominates everything we do. But the problem with fasting is that we're tempted to turn it into another soul-killing law.
There's a time to feast and a time to fast. It's the disciplined person who can feast when feasting is called for and fast when fasting is called for. In fact, the glutton and the extreme ascetic have exactly the same problem. They cannot live appropriately.
So why should we fast? In the Sermon on the Mount, Jesus said, "When you pray," "When you give," "When you fast." "When," not "If." He assumed we'd do these things and instructed us how. We accept giving as a Christian discipline, but not fasting. It may be that in an affluent culture it's less sacrificial to give money than to fast.
We cover up what's inside us with food and other good things, but in fasting these come to the surface. If we're hungry, we don't feel good, and pretty soon we'd do just about anything to feel good—that should not control us.
When you fast, pray, "Lord, reveal what is inside me." Your physical defenses are down when you fast, churning up negative emotions, such as anger, pride, fear, hostility, bitterness, and greed. If we face these sins during fasting, we have a chance to deal with them before they spill over onto other people.