Rabbi Menachem Schneerson, the late Lubavitcher Rebbe, has written that the basic theme of Pesakh is that it "enables us to undergo a personal exodus from Egypt by transcending our individual limits." 

By eating the matzah, he says, we internalize -- quite literally -- this experience. Matzah is the "bread of affliction," the bread of the poor, of people humble and without arrogance. 

By contrast, Schneerson adds, leavening "becomes bloated as it rises, symbolizes self-inflated egotism and pride." As such, one might say the khametz (wheat, barley, rye, spelt, and oats -- expand when contact water) is a physical embodiment of the inclination to evil (yetzer ha-rah).