The Talmud (Sukkah 28b) expresses the primary mitzvah of the Sukkot holiday as “teishvu k’ein t’duru” – literally translated, “to sit [in the sukkah] as if it’s your [primary] dwelling.” “Teishvu” comes from the linguistic root that means, “sit,” “dwell” or “settle” and in its latter meaning can refer to settling land or settling the mind.
Rav Avraham Yitzchok Kook makes the connection between settling the agitated mind and sitting in the sukkah by teaching that when your mind is unsettled, he advises a visit to a place of nature to experience “the tranquility that wells up from the goodness of God that fills all of existence: The songs of the birds as they perch in the trees, the views of the beauty of the Carmel and of the Sharon with their pleasant flowers, and the sweet smell of the lilies and the fruit in God’s garden on earth that He gave to all humanity.
They return one’s mind [da’at] to its natural state after being distanced from it by culture and society. And in returning to one’s source in nature, the creations of God’s hands, all the natural pleasantness of one’s soul returns to him, including feelings of sanctity and ascendancy of spirit that connects to the Almighty, as well as good character.”
When we sit in a sukkah, we remove the walls that separate us from nature and access the experiences that settle the mind. No wonder Sukkot is called zman simchateinu. (From the Mussar Institute)