It is a Mitzvah to eat on the eve of Yom Kippur and to increase the amount one normally consumes (Shulchan Arukh, Orach Chaim 604:1). Seemingly, it would appear to be more appropriate to fill oneself with fear and trembling than food. What reason could there be for joyful eating and drinking at such a time?
Rabbi Moshe Cordovero explains that we rejoice in anticipation of fulfilling the commandment of repentance on Yom Kippur, for it is fitting that we rejoice greatly in our fulfillment of each and every Torah commandment. All the more so when it comes to such an important commandment as repentance. But because repentance, by its very nature involves grief, regret, confession, and a firm resolution to improve, it is impossible, while involved in the act of repentance, to rejoice openly.
Therefore, the Torah commands us to express joy through food and drink before Yom Kippur and, in this spirit, to enter this sacred day – a day wherein God has paved the way for us to return to Him in repentance.