Rav Kook points out that just as most of the tree is roots, trunk, branches and leaves, despite the fact that the goal is the fruit, similarly most of life is preparations (e.g. eating, dress, work, cooking, raising children), and often one doesn't even achieve his goal. Does that mean that he wasted his life?! The basic question is, if most of life is "tree", and I want to have meaning in life, I have no choice but to find a way to have "taste" in the tree, not just in the fruit. This was the original plan in Eden, and is meant to be the ideal lifestyle, as expressed in the Etrog where the tree tastes like her fruit. Eretz Yisrael is likened to Sukkot, where even the secular/mundane/"tree" has meaning/taste/holiness, and it's all a mitzva.
Sukkot is the climax of agricultural success, but also brings with it the anxiety & nervousness regarding the upcoming season. The Sukkah represents the clouds which precede the rainy season, & are the time when God judges us regarding rain. Cherophobia is a common problem where happiness & accomplishment are accompanied by fear: After I've achieved my goals, there's a feeling of emptiness- What do I do now?!". Also, we ask like Kohelet, "Why am I still not happy?!". There's also the fear that I may lose or someone will take away what I've achieved. Also the fear "what if people reveal that I'm really not so talented?". Sukkot are termed in kabbala: "The Shade of Emuna", that only God has absolute success, & only He can provide security regarding past & future.
Sukkot is a complex time of the year. Within the holiday season, it is at the end of the season of the regalim, which starts with Pesach, in the first month of the Jewish calendar (Tishrei is the seventh, not the first month). In the agricultural year, which is so central to the calendar and the holidays, as described by the Torah (see Shemot 34:22), Sukkot is at (/near) the end of the harvest season, i.e., the end of the agricultural year. Therefore, at least if one has an optimistic perspective, this is a time of celebration, and we call it “z’man simchatenu.” This is a stark contrast to the mood of the recent Yamim Nora’im, which focus on fear and trepidation. Many explain Sukkot as a natural return to a more optimistic tone after hopefully succeeding in the spiritual “heavy lifting” of Yom Kippur.
The Vilna Gaon famously pointed out that there are two mitzvot that one fulfills with his whole body – living in Israel and living in a sukka. Of course this can be a cute physical observation, that there is no specific part of the body to which the mitzva applies. However, it is more likely that the intention is that these mitzvot apply to one’s whole persona.