On Sukkos, we say in the nighttime Kiddush that it is זמן שמחתינו, the time of our joy. The Zohar HaKadosh when talking about this special Yom Tov describes the sukkah as צלא דמהימנותא, the shade of emunah/faith. Seeing that the main mitzva of the full seven days of Sukkos is the dwelling in a sukkah, while taking the arba minim (four species of plants waved around) is de’oraisa only a mitzvah for one day, it must be the mitzva of simcha is intrinsically linked to the mitzva to dwell in a sukkah. It’s possible to say from this connection that emunah and simcha have a deep spiritual relationship, showing that the truest simcha is only attainable through complete emunah, that is attaining da’as Hashem (knowledge of God) to the furthest degree we can.