Why Fast?
The decision to fast is usually made for one of two reasons. There is either a compelling health (diet, detox, medical, etc.) or spiritual need. Many people wonder what will happen if you don't eat or restrict your intake of food. If you are curious about that you can read a little primer about hunger, nourishment, and digestion.
Medical and Health Related Fasting
There are a number of non-spiritually guided reasons for fasting:
Religious and Spiritual Fasting
Fasting is part of the traditions of (you can read more about each fasting tradition by clicking on the links):
The reasons for and manner of fasting are quite different in each tradition. You can learn more about each faith and its fasting practices here. Spiritual fasting has its roots in mysticism and other practices that have been a part of many primitive cultures (Origins of Fasting).
Political Motivations
Fasting can be used as a form of civil disobedience or protest. Hunger strikes, for example, are often water only fasts. Numerous hunger strikes and protest fasts have been important in garnering attention for causes, stimulating public sympathy, and altering government policies. Check out more about Protest Fasting.