Eating is usually a pretty selfish act. It does not have to be that way. One can eat with the intention that the food will give him strength to serve God(bechol derachecha da'ehu). It is known that certain righteous individuals eat with very high kabbalistic intentions (leyached yichudim, leha'alot nitzotzot, etc.). They eat in order to say the blessing before eating, and not the other way around. Most people are not on that level. The stomach searches in vain for food to digest and lets us know that we are hungry. It demands our attention. We eat something, and our stomach is satisfied. Sometimes we are not even hungry. We see some tasty food, and we devour it because we enjoy its savor. This is just one manifestation of the trait of selfishness that dwells within us.
The fast enables us to free ourselves from our selfishness. Our self-serving instinct to satisfy our hunger and thirst is put on hold. The mind is now open to consider things that we may not have the time to think of when we are busy fulfilling our bodily desires. We remember that there are people that are hungry on a regular basis. What have we done to help them? We remember the terrible decree that Haman devised to wipe out the Jewish people, and how this decree helped us to become unified and to return to God. What have we done lately in terms of our relation to God? Are we getting closer, or are we distancing ourselves from Him? Where are we in relation to the Jewish people? Are we divisive rabble rousers, or have we encouraged unity? Are we sensitive to the suffering of others, or are we indifferent to problems as long as they do not occur in our own backyard?
These are just a few of the things that we can consider while our head aches and our empty stomach growls.