The mitzvah of simcha appears in parshas Re'eh in many different contexts. The parsha begins with bringing korbanos to Yerushalayim and rejoicing by partaking of them with our families. Later in the parsha we are commanded to bring ma'aser sheini to Yerushalayim and to eat it b'simcha. Parshas Re'eh concludes with rejoicing on the shalosh regalim. As simcha is so integral to these various mitzvos, we must understand what the nature of simcha is.
The Torah constantly mentions simcha in the context of being lifnei Hashem - in the presence of Hashem (Devarim 12:7, 12:12, 15:26, 16:11). Rav Soloveitchik explains that there is an intrinsic connection between simcha and being in the presence of Hashem. It is being in the presence of Hashem that obligates us to rejoice. The visits to Yerushalayim for the shalosh regalim, eating korbanos, and ma'aser sheini are times when we are physically in the presence of Hashem. There are other times where we stand spiritually before Hashem and we are also required to be in a state of simcha. According to many opinions there is a mitzvah of simcha on Rosh Hashana and Yom Kippur. It is for this reason that many of the laws of mourning are suspended during these days as they are during the shalosh regalim. Even though there is no requirement to visit the beis hamikdash or to eat korbanos on these days, the teshuva that we perform on Rosh Hashaha and Yom Kippur places us spiritually lifnei Hashem and as such requires us to be in a state of joy.
The Rambam (Hil. Megillah 2:17) describes at length the highest level of simcha one can attain. On Purim we are required to fulfill three mitzvos to enhance the simcha of the day. We eat a festive meal, we send mishloach mannos and we give mattanos l'evyonim. Although each of these obligations is a part of the simcha of Purim, one should concentrate his time and resources primarily on mattanos l'evyonim. The Rambam explains that by doing so one follows in the ways of Hashem Who brings joy to those who are downtrodden. The ultimate expression of being in the presence of Hashem is to imitate Him. By assisting the poor we are not only enabling them to rejoice but we are enhancing our own simcha. By following the example of Hashem, we add another dimension to being lifnei Hashem.
The concluding pesukim of parshas Re'eh disucss simcha on the shalosh regalim. In terms of halachic practice, the obligation of simcha is equal on all of the regalim. Nevertheless, from the pesukim themselves it would appear that the Torah is differentiating between the regalim. On Pesach the mitzvah of simcha is not mentioned explicitly. Chazal derive its existence from the comparison of Pesach to the other regalim. Although simcha is mentioned on Shavuos, it only appears one time in the pesukim. On Succos we are commanded to rejoice and this mitzvah is repeated with the phrase "v'hoyisa ach someach". In Vayikra (23:40) we are commanded again to rejoice on Succos for seven days. It is this emphasis on simcha on Succos that led Chazal to define the essence of Succos as being zman simchaseinu. What is the significance of Succos being the highest expression of simcha?
Chazal compare our relationship with Hashem to a marriage. The shalosh regalim correspond to the three stages of celebration that accompany a wedding. The kiddushin is celebrated on Pesach as Hashem chose us to be His nation at this time. The nissuin took place on Shavuos, as Chazal compare ma'amad Har Sinai to a chuppah. Succos corresponds to the week of sheva berachos celebration. The simcha between a chassan and kallah barely begins at kiddushin, since the couple does not yet know each other as husband and wife. The simcha progresses slightly at the chuppah, but the true simcha only exists later as they become closer to one another. So too is our relationship with Hashem. On Pesach we have just met Hashem and our joy at being in His presence is limited. On Shavuos as we become closer our joy grows. Only once we are already in His presence, like a married husband and wife, is our joy complete. Only on Succos can the Torah emphasize simcha to such an extent that it defines the essence of the day.
Perhaps there is an additional dimension of Succos that brings an overflow of joy. Not only is Succos the culmination of the shalosh regalim, it is also the conclusion of the Yomim Noriam season. On Succos we celebrate the accomplishments of the Yomi Noraim. The most critical aspect of Rosh Hashana and Yom Kippur is the renewal of our relationship with Hashem. The Rambam (Hil. Teshuva 7:7) describes the greatness of teshuva. Prior to teshuva a person is distant from Hashem. Teshuva brings him into the presence of Hashem. It is this realization that brings about the immense joy of Succos. Since simcha and lifnei Hashem go together, the closer we are to Hashem the more intense our simcha becomes. As we stand before Rosh Chodesh Elul looking forward to and preparing for the glorious days of chodesh Tishrei, let us set our goal to be coming closer to Hashem. Let us focus on spiritually being in His presence, following in His ways of bringing joy to others, and thereby merit to be lifnei Hashem in the physical sense as well, rejoicing in the beis hamikdash, b'meheira b'yamienu.