One of the reasons Chazal stressed that we should eat well on Erev Yom Kippur is so that we will be in a good mood and be willing to forgive each other. 

The Sfas Emes writes: “I think the atonement takes place more on the 9th day than on the 10th. Because on the 9th day, the Jewish nation makes peace with one another. As Chazal say, if you sinned to your fellow man, you aren’t forgiven before receiving his forgiveness. When one fasts, he is angry, and that divides people. But when one eats and drinks, he has a happy heart, and all Yidden unite.”