Question: It is written that we have to be joyous on Sukkot. What does that mean?
Answer: Joy is an effect of attainment and unification; it is joy from bringing contentment to the Creator. There is the World of Infinity, the vessel of the Creator, which is empty. I fill the Creator through the group, and this act of fulfilling Him causes me to feel joy.
At first, I felt empty and demanded fulfillment from the Creator. However, thanks to studying with the group, I realized that fulfillment comes solely from fulfilling the Creator. I am filled with bestowal to the Creator in the same way as a mother is filled with joy when her child is fulfilled.
Ultimately, it makes no difference to me whether I give or receive. The goal is to be fulfilled which I can achieve solely by way of fulfilling Him. When I fulfill Him, I fulfill myself. However, if I fulfill Him for self gratification, it is egoism (Klipa, shell); it’s similar to working for a reward.
However, if I fulfill myself in order to fulfill Him, it is bestowal (holiness). If I fulfill Him in order to fulfill myself, it is reception (impurity).
Diagram: Impurity: 1-2) I fill Him, 3-4) in order to get filled myself. Holiness: I) I raise MAN to Him, II) to make Him fill me, III) so that by filling myself, IV) I could fulfill Him.
In either case, we are both fulfilled. Still, everything is determined by the intention: For whom do I perform these actions? If there is love between us, the more I get filled, the more I fill Him. In our world, there are no other models for this than a mother and her child. 
From the 1st part of the Daily Kabbalah Lesson 9/22/10, “The Agenda of the Assembly”