On the first day of Sukkos, the Torah commands us to fulfill many mitzvos: sukkah, lulav, esrog, simcha, etc.  On the rest of the days of Yom Tov, there are just two mitzvos:  sukkah and simcha.  But on Shemini Atzeres the Torah is very clear.  There is only one mitzvah that we must perform with every fiber of our being –“And you shall be only joyous.” ~ The Chasam Sofer