Why Rejoice on Erev Yom Kippur?

Our sages teach that one who eats on Erev Yom Kippur is considered as if he had actually fasted on both the ninth and the tenth of the month.
Someone once approached Rav Pinchas of Koritz, zt”l, and asked, “Why is eating on Erev Yom Kippur as great as fasting?”
The Rav explained, “Eating a festive meal today is our way of celebrating Hashem’s kindness in forgiving our sins on Yom Kippur, and this is how we demonstrate our faith that Hashem is kind and forgiving. Since it is in the merit of this faith that we are forgiven, our eating on this precious day can achieve so much.”
Rav Meir Marom, zt”l, was the Rav in Kovrin and had been a student of Rav Moshe of Kovrin, zt”l, a great Chassidic master. Aside from Chassidim, there were also many German Jews living in Kovrin who occasionally displayed some curiousity about the avodos of the Chassidim living among them. One Erev Yom Kippur, a local German Jew visited the Rav to see how he conducted his meal. The visitor was surprised to find the Rav in a state of euphoria. It was as if the Rav thought it was Simchas Torah instead of Erev Yom Kippur! This seemed strange to the German Jew, and not in keeping with the solemnity of the day. Although the visitor did not say what was on his mind, Rav Meir discerned his confusion.
“You wonder why I am so joyous?” asked the Rav. “Why shouldn’t I be joyous? Let me tell you a parable to help you understand. Once there was a king who announced he was clearing out his dungeons. All prisoners were to be granted full amnesty. Whom do you suppose this proclamation made the happiest? Obviously, the very worst perpetrator! That is why I am so festive. When the biggest sinner hears that all will be pardoned tomorrow, he certainly feels joyful anticipation and gratitude!”