http://etzion.org.il/en/symbolism-chametz-0 (Rabbi Ezra Bick)
http://etzion.org.il/en/symbolism-chametz (Rabbi Alex Israel)
(3) You shall not eat anything leavened with it; for seven days thereafter you shall eat unleavened bread, bread of distress—for you departed from the land of Egypt hurriedly—so that you may remember the day of your departure from the land of Egypt as long as you live. (4) For seven days no leaven shall be found with you in all your territory, and none of the flesh of what you slaughter on the evening of the first day shall be left until morning.
Idolotry
(ד) ומפני ש'עובדי עבודה זרה' לא היו מקריבים לחם אלא שאור והיו בוחרים להקריב הענינים המתוקים ומלחלחים קרבניהם בדבש - כמו שהוא מפורסם בספרים אשר סיפרתי לך; וכן לא תמצא בדבר מקרבנותם מלח - מפני זה הזהיר האלוה מהקריב "כל שאור וכל דבש" וציוה בהתמדת המלח "על כל קרבנך תקריב מלח":
(4) The idolaters did not offer any other bread but leavened, and chose sweet things for their sacrifices, which they seasoned with honey, as is fully described in the books which I named before: but salt is not mentioned in any of their sacrifices. Our Law therefore forbade us to offer leaven or honey, and commanded us to have salt in every sacrifice: "With all thine offerings thou shalt offer salt" (Lev. 2:13).
See Haggada Shelema by R. Menachem Kasher - Appendix #7 - where he draws a series of halakhic parallels between the laws of idolatry and the laws of chametz: 1. The prohibition of even seeing it. 2. The requirement to burn it, to eradicate its existence. 3. The prohibition not only of eating but of any manner of benefit from it. 4. The prohibition of even the most minuscule particle. This is true for idolatry and chametz, but is not true for any other prohibitions in Jewish law.]
Evil Inclination
ר' אלכסנדרי ...ור' אלכסנדרי בתר דמצלי אמר הכי רבון העולמים גלוי וידוע לפניך שרצוננו לעשות רצונך ומי מעכב שאור שבעיסה
After his prayer... Rabbi Alexandri prayed, he would say the following:
Master of the Universe, it is revealed and known before You
that our will is to perform Your will, and what prevents us?
On the one hand, the yeast in the dough, the evil inclination that is within every person;
Artificial Sweetness
"Scripture calls all substances which have a sweetening effect 'honey' because honey is the classic sweetener. As for leaven, se'or is a human manipulation of the natural state of God's universe. It is an exercise of human machination. God warns us not to use these in the Temple, as the nearer one finds oneself to God, the less room there is for human ingenuity."
"LEAVEN MAY NOT BE EATEN (on Passover) ... matza takes no advantage of the human technological ingenuity and creativity which allows man to raise the dough more than simple flour and water which are created by God. Chametz is the epitome of human involvement in nature. Thus, non-leaven is the symbol of the survival and ongoing existence of the Jewish People as they survive solely through the spirit of God."
ואפילו בעת שהיינו בא״י ומנהיגים מלוכה ומלחמת תנופה כדרך תחבולות האדם. כל התחבולות לא היו אלא כדי שיהא נראה כמעשה טבע לפי שאין הדור ראוי לנס נגלה אבל העיקר היה תלוי אם ה׳ היה יוצא בצבאותינו בהשגחה פרטית. וכש״כ בעת שישראל מפוזרים בעמים ידוע אשר אך השגחת ה׳ ורוח היהודית שבהם מאחדם ומקיימם בעולם
The Netziv - Rabbi Naftali Tzvi Berlin (Russia: Volozhin 1817-1893)
Rabbi Alex Israel
The Netziv sees the raising agent (se'or) and its product chametz as an expression of human interference in nature. Conversely, matza is an expression of the world controlled exclusively by God. Man's discovery that sourdough could cause bread to rise was a technological breakthrough in food technology, a classic human manipulation of the elements of nature. Most of the time, the advancement of civilization in this manner is welcomed by the Torah. This creativity and ingenuity is depicted as the "divine" in man, his "image of God." Not only God, but man too, can create!
However, in a place where God's presence manifests itself most intensely - in the Temple - there is no place for man's creative spirit. In the Temple, man is dwarfed by God. The altar of God is no place for human food technology. On the altar, we dedicate all the elements of God's world - the animal, vegetable and mineral - recognizing and demonstrating that God is the source of them all. Chametz - the product of human manufacture - has no place on the altar of God. It would be presumptuous.
Similarly, on the festival of the miraculous birth of our nation, when an entire nation walked to their freedom away from a tyrannical regime, without lifting a finger of their own, we commemorate the power of God. God performed the Ten Plagues and we had no hand in them. At the Red Sea, the people stood huddled together, frightened, terrified in the face of the imposing Egyptian army. Moses instructed them:
THE BEGINNING OF THE ROAD
One final approach comes from a contemporary scholar - Rav Yoel Bin Nun. He notes that there are occasions when we DO bring leaven to the Temple (although it is not offered up on the altar itself). On Shavuot - Pentecost - we bring two loaves of bread to the Temple (Vayikra 23:17). In the thanksgiving offering (a variation of the peace offering - shelamim), three types of loaves are brought to the Temple: unleavened wafers - like our matzot; unleavened loaves - like pita; and leavened loaves - like our bread.
What is the symbolism that leaven and unleaven represent in the Temple? Leaven represents fulfillment, a process which has gone its due course. The ultimate and supreme form of flour and water is a leavened loaf. Unleaven, on the other hand, is "not yet" what it aspires to be; it figuratively represents the beginning of a yet-unfulfilled process. It is presently immature and unripe. It is in the early stages of a journey.
The altar of God is not a place for leaven. Before God, we are all rough around the edges. We all have a way to go in reaching our own personal destiny. We have faults, room for improvement. We cannot express ourselves before God represented by the symbol of leaven, for we are at the beginning of a journey. We are the unleavened, still traveling on the tortuous road that is human and religious betterment. So when do we bring leaven to the Temple?
Shavuot is the Festival of Weeks. It is also the festival designated as the time to bring first fruits from the new crop in the Land of Israel to the Temple. Shavuot is linked to Pesach by the Omer. We count seven cycles of seven days from Pesach and then we celebrate Shavuot. Pesach is the start of a process; Shavuot is the end. On Pesach, we remove all leaven and eat only unleavened bread. On Shavuot, we bring loaves of leaven. It is a question of a process.
On Pesach we had our freedom. One might revel in the euphoria of freedom and imagine that this is it. We have reached our goal, we have achieved independence. In response, God tells us to eat only matza - unleavened, unfulfilled bread - for seven days. Pesach begins a process. It is a cause for celebration, but it is only the start. The goal comes seven weeks later in a festival which celebrates two things. First, it celebrates our spiritual challenge. It is the festival of the giving of the Torah. The Torah embodies our challenge, our goal and our destiny. Second, it is the festival of the Land of Israel. On Shavuot, we bring the first fruits and stress the idea that the goal is creating "a kingdom of priests and a holy nation" - a people in the land of God living the moral law of God.
On Pesach we mark the beginning, the unleavened. Shavuot expresses everything that embodies our national goal. It is therefore marked by the bringing of fully leavened, fulfilled bread.
Likewise, in the thanksgiving sacrifice where I express my release from a life-threatening situation, when saved from a serious illness or the like, we bring three loaves. The totally flat matza wafer, the unleavened pita loaf, and the fully risen loaf of bread. The offering describes the journey from the depths of desperation to the heights of health and life. The symbolism of this offering tells of the role that God played in breathing life into a seemingly hopeless, flat situation, granting hope and salvation. That is the role of leaven and unleaven in the thanksgiving offering. Unleaven is the beginning of the process and it leads to fulfillment in the form of the leaven.