A Clean Slate.
Vayelech (Deuteronomy 31 )
Yom Kippur is the day the Jewish nation received the second set of tablets, reestablishing their intimate connection with God 80 days after the sin of the golden calf. Ever since, this day has been a day on which every Jew is given the opportunity to rectify any mistakes he has made.
It is interesting that the Talmud gives us advice on how to get this clean slate and reconnect with God on any day of the year. We are told that anyone who can drop his pettiness and overlook most of life's perceived injustices will be treated accordingly by God, Who will reciprocate and drop all claims against him. This may be why we start the Yom Kippur service with the words, "Light is sown for the righteous and someone with an upright heart will experience joy."
The "upright heart" refers to someone who can excuse people for their mistakes, trying never to judge them or to be critical of them, but rather to try to see only the goodness and beauty in people. A person should never think he is the only one in the world who deserves respect and that he is the only one who is always right. We should bend over backwards to be gracious to others and forgive them for their mistakes, and God will reciprocate and forgive us.
The level of "joy" will be commensurate with the extent that we can cultivate this "upright heart".
May God's "light" shine on us all.