On the first day of Sukkos, the Torah commands us to fulfill many mitzvos: sukkah, lulav, esrog, simchah, etc.

On the rest of the days of Yom Tov, there are just two mitzvos (midiOraysa): sukkah and simchah.

But on Shemini Atzeres, the Torah is very clear.  There is only one mitzvah that we must perform with every fiber of our being: “Vehayita Ach Sameach – And you shall be only joyous.”

~ The Chasam Sofer