There is a concept connected to the Succah that is known as , צלא דמהימנותא, “the shadow of faith.” This expresses the ultimate purpose of sitting in a Succa for seven days.
The way of all men is different from that of Israel. It is the way of all men to feel secure and unafraid under the shelter of their own roofs. When they leave their homes, their sense of security is diminished, and they begin to feel free.
Israel, however, is different. While in their own homes all year, they are apprehensive, that they become haughty at heart, and forget their Maker.
When Succot comes, and they leave their homes and come under the shadow of their Succot, their hearts are filled with trust, faith, and joy-for now they are shielded not by the protection of their roofs, but by the shadow of their faith and trust in G-d.
This is not only the message of Succot, but it is meant to be a constant reminder of where real “security” originates. It is not a comfortable home, or a large bank account, but every breath that we breath, and our very existence, depends entirely on our faith and dependence on Hashem.
This also explains why a Succa is invalidated if it has more sun than shade. Not only must the Succa be a “shadow of faith,” but so must our devotion in serving G-d, must be a “shadow of faith.”