Shemini Atzeres. We have been eating and drinking for days. We don't have that same desire for the food and drink. Thus, this is the time, to gather all the Kedusha that we traversed during these holy days and do it all completely L'shem Hashem.
Shemini Atzeres 5672-It says in Shir Hashirim Rabbah 7 that Atzeres, Shemini Atzeres, is supposed to be fifty days after Sukkos, just as Atzeres, Shavuos, is fifty days after Pesach. The reason that it is fifty days is Kneged the נ' שערי בינה, the fifty gates of Binah. The reason that by Shavuos there are fifty days after Pesach until Shavuos is because there was no Hisororos from down here - Klal Yisroel were bare with no Mitzvos, and they needed everything to come from above. It was all with the great Chesed of Hakodosh Boruch Hu. However, by Shemini Atzeres, Klal Yisroel are coming with all the Mitzvos they performed with the Sukkah, Lulav, etc... There is Hisroros from down here, and thus they can reach a higher level quicker, and be Zoche to have the Yom Tov of Shemini Atzeres. The Avodah of Shemini Atzeres is to take all that Hisororos that we just had and put it into action. The Yom Tov of Shemini Atzeres is to tell us that we should not be content with all that we did over Yom Tov, but now we need to go back into our homes and take that Kedusha and draw it into our daily lives. We need to take it with us throughout the year.
Shemini Atzeres 5672-On Shemini Atzeres, we leave the Sukkah and we rejoice with Shemini Atzeres. The Gemara in the beginning of Meseches Sukkah says that the Torah tells us that we are to leave our permanent dwelling and dwell in a temporary dwelling for seven days. My father, the Avnei Nezer said that the world of nature that we live in is called the permanent world, for it occurs regularly. What happens by way of miracles, that is called a temporary dwelling, for it only occurs on occasion. Although the Maharal says that these miracles are not happenstance, and do not occur randomly, nonetheless, they only occur sometimes, and as such are called temporary, as this world is not worthy of having it regularly. Odom Harishon, before he sinned, he was Zoche