Why do we fast?
Not eating is a way of using our mealtimes to glorify God who is our true nourishment. Christ Himself fasted, as the gospels record, and He told His followers to do the same. In that way, many Church Fathers taught, the Fall, which was caused by eating out of turn, is reversed.
Fasting is the necessary complement to feasting. If feasting is a daily event for us, we are no longer celebrating the Lord when we feast: simply satisfying our desires. If we give ourselves over to the rhythm of feasting and fasting, then our table actually become a place where God is glorified. All our eating becomes sacramental: a means of communion with the Life-Giver.
There are two ways to fast practiced in our Church. Sometimes they are practiced separately; other times they are practiced in tandem. The first type is the total fast in which we simply do not eat or drink anything. We fast in this way to heighten our sense of preparation or expectation, looking forward to what is to come. In the liturgical tradition a total fast is the ultimate preparation for a feast. This we find on the eves of Christmas and Epiphany (Theophany). We also fast in this way before receiving the Eucharist to help us anticipate the heavenly Bread we are to receive. It is a necessary condition for Holy Communion.
The second kind of fast has been called an ascetical fast, in which we refrain from eating certain foods while eating others. We may remember giving up candy or sweets for Lent as children: this was an ascetical fast. In the Byzantine tradition it is generally meat and dairy products which are subject to the ascetical fast at one time or another.
The purpose of ascetical fasting is to teach us that we don't need that steak or ice cream: we live, as Christ said, not by bread alone. We need Him This kind of fasting practiced over a long period of time helps us to develop a kind of spiritual strength. By becoming less dependent on satisfying physical needs, our spiritual muscles harden. And so we can do without this or that pleasure, even for a long time, because we know we have the One we really need.
When we begin to fast in this way, the first thing we realize is how enslaved we have become to pizza or cheeseburgers. We discover that we are not as spiritual as we might like to think. We find ourselves more dependent on Burger King than on the King of all! This is the first lesson of fasting. If we persevere through that initial sensation, we will find that by far and large we can get along without this or that, and that the discomfort of fasting serves to heighten our awareness of the Lord's presence.
Vast numbers of people fast in both these ways for health or beauty: we need not fear doing it for an experience of God.