By Rabbi Aharon Seleh:
On Sukkot, there was a special mitzvah of drawing water and pouring it on the mizbe’ach. This was accompanied with an intense Simcha. So much so, that the Gemara says if one never experienced it, one never saw real Simcha in their life. Harav Yitzhak Yosef explains that many did teshuva on Yom Kippur out of fear, which cannot erase intentional sins. It just downgrades them to unintentional sins. That’s why we have tashlich, where we throw out all the unintentional sins. But by Sukkot, with all the happiness, our teshuva is out of love. Teshuva from love turns our sins into merits, at which point we go back to the water to draw up the sins-turned-merit.
Furthermore, water represents love. The mitzvah to pour water on the mizbe’ach is to show that all our love is for Hashem. We sit in a Sukkah to disconnect from the world and the love of materialism. We shake the etrog, which represents the heart, lulav the spine…to show all our love and desire is for Hashem.