The Lord says, "When you fast . . ." (Mt. 6)
Fasting is a practice in the Church not isolated from prayer and almsgiving. All of these practices are signs of our mortality and of our need to die to sinful desires, that is, our need to repent. Fasting is a self-discipline that helps to keep our focus on our need for God's forgiveness. Repentance and forgiveness are daily in the baptismal life, though Lent offers us the opportunity to be reminded of and be catechized in disciplines that we are usually too busy to even contemplate. Usually, it is our own busy-ness that becomes the excuse and for which we also need to repent.
Fasting, prayer and almsgiving are not practices that one does to get the attention of God and others. They are done as a means of preparing for receiving the Eucharist, preparing for the paschal Feast, preparing for the Life to come. Praying the daily office, individual confession and absolution and helping the needy are all disciplines of the Christian faith, places where mercy and daily life connect.
These disciplines are good works wrought out of the forgiveness that we are given and receive in Christ. His own Incarnation, humility, service to God and others and death on the Cross bear us in all of our infirmities and weaknesses. Even as the Son of Man humbles Himself so He lifts us up. When we are weak, then we are strong.
Just as repentance is not complete without forgiveness, so the life of discipline is not complete without God's grace and mercy in Christ. The benefits of the Cross - Christ Himself - is our reward in His Body and Blood. The fast is made complete in the Feast which He prepares for us and through which He gives us strength and through which we have Life.