A day of fasting in the Jewish calendar – such as this Sunday's Fast of Tammuz – is not one of dieting, but rather a day of introspection and teshuva (contrition and remorse). We are not fasting for something far-removed from ourselves, but rather for our situation this very day.
Fast days are occasion for introspection and repentance. After all, we are not fasting over the distant, unrelated past; we are fasting in response to our own present situation. How is it that instead of mourning we remain complacent and indifferent?