Answer: Because Chametz symbolizes the connection between flour and water. Flour symbolizes the attribute of the egoistic desire and water symbolizes the attribute of the altruistic desire.
Therefore, on Passover we are at final stage of the domination of the ego and we begin to feel that egoism controls us in everything since we lack the attribute of Hassadim, the attribute of Hesed, the attribute of the water, which is what the Matzah symbolizes.
Matzah is the connection between flour and water. They are mixed for a very precise period of time, for 18 minutes, so that the flour will not begin to rise. If they are mixed too long, we have to constantly knead the dough without stopping…
This process symbolizes our work with egoism, the process in which we have to keep working with it, giving it new forms over and over again, moving to altruism. And then it is possible.
But in principle, it is impossible to work with the Chametz for more than 18 minutes, which means that the connection has to be minimal, just as it appears in our egoism. What is altruism? That we want to show good intentions toward someone so the ego will benefit from it? The Matzah symbolizes the lack of water, the lack of Hassadim, the lack of Hesed, in our egoistic attitude, and so it is called the “bread of poverty,” which means the poverty of a wide heart, in the openness between people.
It is customary to eat Matzah on Passover eve because it is on this night that we reveal to what extent we don’t have the attribute of love and bestowal among us, the attribute of Bina, or the attribute of Hesed. We are ready for anything as long as we can exit this egoistic state, and so we eat Matzah on the night of our liberation from egoism.
From KabTV’s “News with Michael Laitman” 4/13/16