Based on Bhagavad Gita Chapter 09, Text 14
Some people think of fasting as a means of achieving mastery over their body. They fast to increase their willpower by refusing to let bodily appetites control them.
As spiritual seekers, if this self-centered motive seduces us, then we miss the devotional benefits of fasting, irrespective of whether our fast fails or succeeds. If we fail to fast due to not having enough willpower, then fasting becomes the cause of self-torture. If we succeed in fasting due to having enough willpower, then fasting becomes the medium for expressing and expanding our false ego.
To avoid this lose-lose dilemma, we need to shift our focus from self-centeredness to Krishna-centeredness. The Bhagavad-gita (09.14) points to this divine focus when it asserts that the great souls complement their resolve for strict vows with constant glorification of Krishna. Concentration on Krishna changes our vision of fasting: instead of seeing it as an opportunity to demonstrate our willpower, we see it as an opportunity to experience the power of the non-material nourishment latent in his remembrance. Actually, Krishna consciousness can satisfy us far more than the best food — at any time and at all times. However, we don’t usually experience this satisfaction. Why? Because to get this satisfaction, we need to pay the price of focusing our consciousness on Krishna exclusively. We have little impetus to pay this price when multiple alternative sources of satisfaction are available for us — as happens on normal days.