Seven days shall you dwell in booths (Leviticus 23:42) ... and you shall only be rejoicing (Deuteronomy 16:15).
Succos is the festival designated as the season of our gladness. Yet the commentaries state that one of the symbolisms of the succah, a temporary hut, is that we dwell in it for seven days to symbolize man's temporary sojourn on earth for his average life span of seven decades (Psalms 90:10).
Human mortality is a rather sobering thought; it is hardly conducive to rejoicing. Most often we do not think about our mortality, and when circumstances force us to face it, we quickly dismiss it from our minds and go on acting as though we will live forever.
How different Torah values are from secular values! The Torah teaches us that there is an eternal life, a wholly spiritual life, whose bliss is far greater than the human mind can imagine. We are placed on this planet for our ephemeral earthly existence only to give us an opportunity to prepare for the eternal life.
The Torah teaches us to enjoy life, and if it restricts some pleasures, it is because we should enjoy life in a manner that befits a human being. Furthermore, our joy of living should not be diminished by the awareness of our mortality, nor need we deny it. The succah - the symbol of our temporary stay on earth - is beautifully decorated, and we enjoy our festive meals therein. Even our temporary existence can be beautiful and happy, and our faith in the eternal life should enhance that happiness.