The mitzvah of rejoicing appears in Parshas Reah in several contexts: eating of korbanos, maaser sheni, and the celebration of Yom Tov must all be performed with joy. Whenever we are commanded to rejoice, we are reminded to include those less fortunate in our celebration. From the words of the Rambam (Hilchos Yom Tov 6:18), it would appear that the requirement to share our joy with others is an integral part of our actual fulfillment of simcha. Giving to others at a time of our joy is not merely a fulfillment of tzedaka, but rather is necessary for our personal joy, to be complete. Why is the simcha necessary for korbanos, maaser sheni, and Yom Tov not fulfilled if others aren't included?
Throughout Parshas Reah whenever simcha is mentioned, the Torah emphasizes that the simcha takes place "lifnei Hashem - in the presence of Hashem." Korbanos and maaser sheni are eaten in Yerushalayim "lifnei Hashem" and the Shalosh Regalim are celebrated "lifnei Hashem" by visiting the Beis HaMikdash. Even Rosh Hashana and Yom Kippur have a dimension of simcha which is associated with being in the presence of Hashem. The Rav explained that although Rosh Hashana and Yom Kippur don't have the physical aspect of "lifnei Hashem" of visiting the Beis HaMikdash, the teshuva process which is associated with the Yomim Noraim is also a form of being in the presence of Hashem. It is the state of being in the presence of Hashem that is the ultimate source of joy.
The Experience of being in Hashem's presence, both physically and spiritually, enables us to refocus on how Hashem acts. Contemplating His actions, we immediately are reminded of how Hashem performs acts of kindness to others. Chazal teach us that ultimately our obligation to perform chessed emanates from our obligation to emulate Hashem's ways. Just as He is a gomel chessed, so too we must become gomlei chessed. The more aware we become of Hashem's traits, the greater our obligation to emulate Him becomes. As we visit Hashem in Yerushalayim to partake of korbanos, maaser sheni, and to celebrate Yom Tov we become even more cognizant of Him and His actions. It is this realization that must propel us to greater heights in of performing acts of chessed. If this experience of "lifnei Hashem" is authentic, it must be accompanied by a greater commitment to share with others.
Standing "lifnei Hashem" creates a dual obligation. It is the source of the mitzva of simcha as well as the source of chessed. If one rejoices by eating korbanos, maaser sheni, and celebrating Yom Tov, but does not share joy with others, that is indicative that his joy does not emanate from being in Hashem's presence. A true experience of being "lifnei Hashem" will inevitably result in a heightened awareness of the needs of others. One who spreads his joy with others has truly experienced the source of joy; he has stood "lifnei Hashem" and learned the lessons of that awesome experience.