Just as a person who is enslaved and will be liberated on a certain day will count each day until he is released, so too we count the days until we receive the Torah. Our normal mood should be one of happiness. On the other hand, Sukkot is seven days. Rosh Hashonah is a day of shaking us out of ways displeasing to Hashem, and Yom Kippur is a day of fasting and awareness of our faults and mistakes. To live a joyful life avoid quarreling.
True joy is only attainable when there is peace among people.
This is symbolized by the four species we take in our hand on Sukkot, and which symbolize the various types of peoples who make up the Jewish people. Holding them together is a sign of true peace and unity. Only when there is togetherness can true joy before Hashem be fulfilled. Sukkot and guests are both reminders of our temporary status. The Chofetz Chaim said that the mitzvah of being hospitable to guests benefits the host in a spiritual way.