The Rambam (Hilchos Megila Ch. 2 Hal. 17) rules that it is better to increase the amount of money spent on matanos laevyonim than to add to the lavishness of one's seudah and mishloach manos. The reason is that there is no greater and more splendid simchah than to gladden the hearts of the poor and downtrodden. Moreover, one who gladdens the unfortunate is compared to Hashem, who revives the spirit of the lowly and the heart of the depressed.
Physical pleasure is necessarily limited. The human body can tolerate only a small amount of meat and wine. Therefore, in order to maximize the mitzvah of simchah on Purim, the added dimension of helping others was included. Sharing with peers, mishloach manos, establishes a joyful spirit of camaraderie. Giving to those who are needy and cannot reciprocate, matanos laevyonim, creates an even higher level of simchah.
Spiritual pleasure on the other hand knows no limits. The soul cannot be satiated (Koheles 6:7). Indeed, even negative insatiable drives are corruptions of the unending desire to serve Hashem. One who loves money can never be fully satisfied with the money he has (ibid. 5:9). The Medrash interprets this to mean that one who loves mitzvos is never content with those he has performed. Why did Chazal feel compelled to explain the verse this way when the simple meaning is so clearly true? The ba'alei mussar explain that Man's never-ending quest for money is inexplicable. It must, therefore be a perversion of the inborn, unfulfillable love for mitzvos.
What is the source of this unquenchable thirst for spiritual accomplishment? The Rambam provides the answer. Hashem revives the spirit of the needy. The human soul is a part of the divine above. Therefore, the human soul, as a part of the infinite, has infinite capacity for fulfillment in the divine enterprise of helping others.
The mitzvah to be happy on Purim is best fulfilled by assisting others and ideally by supporting the needy. As such, mishloach manos is an integral part of the mitzvah of eating and drinking on Purim. Matanos laevyonim, which more closely resembles Hashem's acts of kindness to the downtrodden who cannot reciprocate, is the greatest and most splendid simchah for the person who is able to help.
The rabbinic commandment of how to fulfill the obligation of Purim is patterned after the Torah's requirement to rejoice on the Shalosh Regalim together with servants, orphans, widows, and others who need financial or social assistance (Devarim 16:11). Indeed, the Rambam (Hilchos Yom Tov Ch.6 Hal.18) describes the joy of one who does not help the poor and the embittered yet himself enjoys a festive meal as merely the joy of the stomach. The happiness of mitzvos must include gladdening the hearts of the depressed.
The difference between the two interpersonal mitzvos of Purim can be traced to their very inception. The original observance of Purim in the scattered cities included only mishloach manos (Esther 9:19). Only the subsequent enactment of Mordechai, which included walled cities as well, added matanos laevyonim (ibid. 20-21).
Moreinu Harav Yosef Dov Soloveitchik ZT"L explained this distinction based upon the Ramban's analysis of the historical development of Purim recorded in the Megillah. When Haman threatened the Jews, those who lived in walled cities were in considerably less danger than the rest of their brethren. Therefore, following their deliverance, the walled city dwellers celebrated at the time of the miraculous event (ibid. 18), but not in subsequent years on its anniversary. Only those who were in an immediate danger observed the 14th of Adar as a recurring holiday (ibid. 19).
Later, when Mordechai and his beis din instituted Purim as an official day of joy, they realized that all Jews ought to celebrate. No Jew could rightly say, "Haman would not have attacked us anyway", for Haman was the enemy of all the Jews (ibid. 24).
In most of the world Purim is observed on the 14th of Adar. In ancient walled cities it is celebrated on the 15th. The Ramban asks, why did the Rabbis, who are always concerned about uniformity in religious observance, institute a mitzvah with a built in dichotomy?
The Ramban answers that we celebrate two days to commemorate the original enactment of Purim. However, to emphasize that the Jews in more vulnerable areas experienced a greater miracle, and that they were the ones who observed Purim first, Mordechai saw fit to establish one day for them and a subsequent day for inhabitants of walled cities. And so a dual date for Purim emerged for all generations.
Rav Soloveitchik ZT"L added that this very difference underlies the delay in the observance of matanos laevyonim. Originally, when only the Jews outside walled cities celebrated Purim, the lesson of the common destiny of all Jews was not fully appreciated. Therefore, only the mitzvah of mishloach manos, representing closeness with one's peers, was observed. Mordechai's enactment included the walled cities to emphasize the unity of the Jewish people. To do so, he added not only a different date, but also the mitzvah of matanos laevyonim. Helping the disadvantaged reinforces the notion that all are united and must care for one another.
The importance of Jewish unity on Purim can explain an enigmatic statement attributed to the Arizal. Yom Kippur is called Yom Kippurim, which the Arizal reads as Yom K'-Purim - a day like Purim. Since Yom Kippur is a much holier day than Purim, why is Yom Kippur compared to Purim?
The ba'alei mussar suggest that the statement of the Arizal refers to the interpersonal dimension of these two days. On Yom Kippur, we must seek unity. Forgiveness is contingent upon appeasing a fellow Jew (Yoma 85b). The day is described as one which does not contain hatred, jealousy, or competition (Musaf). The fast must involve sinners, just as the ketores included a foul-smelling spice, chelbena (Krisus 6b). The very name Tzom, fast, is related to Tzamah, a braid which symbolizes the unity of different strands within the Jewish community (Rav Soloveitchik). Purim also stresses togetherness. Haman's statement that the Jews were spread out and divided (ibid. 3:8) reflected our disunity. Esther's exhortation to gather all the Jews of Shushan (ibid. 4:16) teaches that Jews must unite, especially in times of crisis.
The mitzvos of the day reflect the same theme. The Megillah should be read in a large assemblage (Mishnah Berurah Orach Chaim 689 note 16). Mishloach manos and, especially, matanos laevyonim reinforce the need for Jews to assist their friends and, especially, the disadvantaged.
The Arizal maintains that the unity of Yom Kippur exists in an unnatural state. In the absence of work and physical pleasure, Jewish unity is more readily achieved. As such, it is hardly a precedent for the rest of the year.
In this sense, Purim is greater than Yom Kippur. Work is permitted and eating and drinking are mandatory. Unity under such circumstances is a greater accomplishment and a better example for other days as well.
Purim and Yom Kippur share another common factor. They are both days of kabbalas haTorah. The second luchos were given on Yom Kippur (Rashi Shmos 34:29). And the second, compelling acceptance of the Torah took place during the story of Purim (Shabbos 88a).
This is no coincidence. The Torah can be given only when there is Jewish unity. At the original kabbalas haTorah on Shavuos, the singular form (VaYechan) is used in reference to Am Yisrael (Shmos 19:2). Rashi explains: as one person with one heart. This prerequisite for receiving the Torah was recreated on Yom Kippur and during the story of Purim with similar results.
Jewish unity cannot possibly be realized without authentic Torah leadership, which requires humility and concern for others. Moshe Rabbeinu was our greatest leader and the humblest of men. He gave us the Torah on Shavuos and Yom Kippur. On Purim, the Torah was reaccepted under leadership of Mordechai.
Although his humility is not stated openly in the Megillah, an insight can be gleaned from the cantillation (ta'amei hamikrah) of the Megillah. Two psukim - 3:12 and 8:9 recount the calling of the scribes to write the king's orders in nearly identical terms. In the first, Haman's commands were written; in the second, Mordechai's.
The emphasis, as denoted by the highest and longest cantillation, known as the pazer, is remarkably different. In Chapter 3, the Pazer is on the word "Haman". In Chapter 8, Mordechai's name is read with the least significant note (munach), and the emphasis of Pazer is reserved for the word "Yehudim," the Jews. This is the fundamental difference between the two types of leaders. Generally, leaders are largely concerned about their own welfare, as was Haman and the Megillah drops us a hint of this by stressing his name as he would have. Authentic Torah leaders, such as Mordechai, however, are devoted to the Jewish people in general, and the people who follow them in particular. Their own needs are downplayed and muted similar to the note on Mordechai's name, and the needs of the Jew are emphasized.