Bo: Zohar HaKodesh: Mitzvah 15 – Concept 113
“For seven days, leaven may not be found in your houses.” (Exodus 12:19) No Chametz may be found in our possession on Pesach. (Rambam, Hilchot Chametz U’Matzah)
This is actually a commandment to protect the joy we have on Pesach. The Children of Israel connected with each other with complete joy on the first night of Pesach, when they offered and ate the Pesach offering in groups.
However, the joy of Pesach was incomplete because they had not yet received the Torah, which is the only source of pure joy. This is why we only sing the complete Hallel on the first day of Pesach.
The joy of Torah is blocked when our spirits rise in arrogance, symbolized by the leavening agents. The Yetzer Harah – The Evil Inclination – which attempts to infect the entire soul, is described as the yeast in the dough.
The Jews ate Matzah on that first night of Pesach, and continued to eat Matzah because their dough did not have time to rise. The Yetzer Harah was blocked. They then went on to eat Manna, Heavenly Bread, free of all yeast. On Shavuot we offer loaves of bread as an offering, because on the day we receive the Torah with joy, we can defeat the “leavening” of the Yetzer Harah..
The joy of Torah and in our service of God will always constrain the “yeast in the dough.” We derive from this law how we must, especially at moments of joy such as Pesach, do whatever we can to make sure that no Chametz is found in our joy. (Based on the Zohar in Emor and Ki Tisa)