Zohar Emor 103b
When people sit in a sukkah, the "shade of faithfulness" -- the shekhinah [the indwelling, feminine divine manifestation] -- spreads Her wings over them and... Abraham, five other righteous ones, and King David make their dwelling with them. And so, people should rejoice with shining countenance every day of the festival, together with these guests [ushpizin] who lodge with them... And we must gladden the poor, for the portion of the guests [one] invite[s] comes from the enjoyment felt by the poor when they eat.
Take good care that your own mind not flee you. I mean the thoughts of your own better self. Just as you guard your money or possessions, making sure no one runs off with them when you are turned away, be even more careful to do the same with your mind, the one you are using to serve God. The best guard is joy to be happy always over the mitzvot you are doing and have done already. The sages spoke of one " who does not stop smiling all day (Brachot 9b)," when he was doing mitzvot. A person who gets to go to the royal palace should be very happy. How much more so if it is the palace of the Divine Ruler. [Each mitzvah is such a palace.] If you rejoice in the mitzvot... you can partake also of this worldly joys...without having your head turned by them. (Liqqutey RaMaL, Breishit, trans. by Rabbi Arthur Green)