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Upon attaining freedom from the physical slavery of Egypt, we rejected the “fleshpots of Egypt” (Shemot 16:3). We rejected the emphasis on physicality, and embrace, as servants of the Lord, a life of simplicity.
Our lives were now “theocentric;” God is at the center, and not our own will and desires. We eat, therefore, not chametz, but matza, “lechem oni,” the simple, poor-man’s bread.
The whole halachic construction and laws and details and minhagim is based on this.