The Purim seudah is a multifaceted mitzvah. Chazal instituted eating and drinking as the vehicles through which we celebrate the miracle of Purim. Although on the surface the seudah appears to be merely a physical form of celebration, there is a spiritual dimension that commemorates the religious revival that occurred at the time of Purim.
Megillas Esther opens with an elaborate description of a feast given by Achashverosh for the entire population of Shushan. This feast acts as a catalyst for the future events in the megillah on two levels. Vashti's refusal to attend the feast leads to her downfall, facilitating Esther's rise to the throne. In addition, Chazal saw this drunken celebration full of immorality and depravity, attended by Jews, as critical in its timing. Achashverosh had erroneously calculated that 70 years had passed since the Jews had been exiled, and because Yirmiyahu's prophecy of the rebuilding of the Beis Hamikdash had not been fulfilled, concluded that it would never be. Achashverosh was celebrating Hashem's apparent absence from the world by drinking wine form the sacred vessels of the first Beis Hamikdash. The Jews' participation in this infamous party, dedicated to the desecration of the Beis Hamikdash, was a sin that sealed the fate of the Jewish people, destining their annihilation at the hands of Haman.
The repentance brought about by Mordechai's leadership revoked the decree. To commemorate these events, Chazal instituted the mitzvah of seudas Purim. This seudah serves as the ultimate correction for the tragic mistake made by the Jewish people in attending Achashverosh's seudah, by eating and drinking while maintaining the highest standards of morality. The seudah would be dedicated to praising Hashem for His presence in the world, as became evident through the miracle of Purim.
There is yet an additional dimension to the Purim seudah. Chazal tell us that the receiving of the Torah was incomplete until Purim. The Torah was originally accepted under duress. Hashem appeared, accompanied by lightning and thunder, following all the revealed miracles of yetzias Mitzrayim, and the Jewish people were so overwhelmed that they were in a sense coerced into accepting the Torah. At the time of Purim, when Hashem's presence in the world could have been questioned, the willful acceptance of the Torah was complete. Halacha requires a special seudah to be eaten on Shavuos to celebrate the first stage of the giving of the Torah. The seudas Purim is the culmination of the seudah of Shavuos.
The Rambam incorporates the mitzvah of mishloach manos as part of the mitzvah of seudas Purim. The Ksav Sofer suggests that the relationship between these two mitzvos stems from the aspect of celebrating kabolas haTorah. A person can eat and drink because of joy or sorrow. Excessive eating and drinking can be used as a way to forget one's agony. When one eats and drinks on Purim to celebrate kabolas haTorah it is unclear whether one is overjoyed by receiving this wonderful gift or is miserable because he is now overburdened by its restrictions. A person's mood can easily be discerned. Only when he is eating with others out of joy does he want to share this joy with others. One who is miserable wants to be left alone. It is the mitzvah of mishloach manos that reveals the true nature of the seudah.
As we eat our Purim seudah let us remember to focus on its spiritual dimension, thereby uplifting our eating and drinking to a joyous service of Hashem.