If the solemnities of our fasting, though frequent, long, and severe, do not serve to put an edge upon devout affections, to quicken prayer, to increase Godly sorrow, and to alter the temper of our minds, and the course of our lives, for the better, they do not at all answer the intention, and God will not accept them as performed to Him.
Author: Matthew Henry Source: A Commentary on the Whole Bible, Funk and Wagnalls, v. 4, p. 1478.
Fasting in the biblical sense is choosing not to partake of food because your spiritual hunger is so deep, you determination in intercession so intense, or your spiritual warfare so demanding that you have temporarily set aside even fleshly needs to give yourself to prayer and meditation.
Fasting, if we conceive of it truly, must not be confined to the question of food and drink; fasting should really be made to include abstinence from anything which is legitimate in and of itself for the sake of some special spiritual purpose. There are many bodily functions which are right and normal and perfectly legitimate, but which for special peculiar reasons in certain circumstances should be controlled. That is fasting.
The purpose of fasting is to loosen to some degree the ties which bind us to the world of material things and our surroundings as a whole, in order that we may concentrate all our spiritual powers upon the unseen and eternal things.
The abstinence is not to be an end in itself but rather for the purpose of being separated to the Lord and to concentrate on godliness. This kind of fasting reduces the influence of our self-will and invites the Holy Spirit to do a more intense work in us.
If you say "I will fast when God lays it on my heart," you never will. You are too cold and indifferent to take the yoke upon you.