Through completing our mitzvot, we bridge the gap between the physical and spiritual worlds. We uplift our universe to make it pure. On Sukkot, we do this with the mitzvah of Etrog: “And you shall take unto yourselves on the first day [of Sukkot] a fruit of a beautiful tree.”
What makes the Etrog beautiful? One aspect is that it exists all year round. It does not appear in the best of times and then vanish. It maintains its consistency; it is permanent. This is a hint to us that what is beautiful is aligned with what is true. The spirit, the neshama – the impact we create in eternity – this is what stays forever. What is beautiful is what is eternal, nothing else.