A Jew should be happy. "Serve the LORD with gladness; come before His presence with singing (Psalms 100:2)." "Because thou didst not serve the LORD thy God with joyfulness, and with gladness of heart, by reason of the abundance of all things; (Deuteronomy 28:47)." And if a Jew is not happy, he is clearly doing something wrong.
Together with this, we are no entering a sad period, the "three weeks". Together with the general feeling of happiness and gratefulness for all we have and all that God has done for us, we devote our thoughts to what is lacking: our Holy Temple. Don't get me wrong; The Temple is always in our prayers! But at this time of year, close to the date of its destruction, the anguish over what we don't have is magnified.
May today's fast help us to find our own faults and to correct them. May we learn to love our fellowmen as ourselves. May we be blessed to see the coming of the Messiah and The Temple rebuilt.