Another difference (from the Lubavitcher Rebbe):
חמץ and מצה both share the same letters, aside from the ה and ח
When someone is haughty (חמץ), he can’t do teshuva because he always blames any wrongdoings on others and he never feels guilty.
The ח only has an opening on the bottom – the only path such a person will go is downward ר”ל.
When someone is humble (מצה), he can always do teshuva, for he is quick to recognize his wrongdoings.
This is symbolized by the ה which has an opening on top to get out of our wrongs and get closer to Hashem.
This Pesach, let’s try to be matzos. Lowering our ישות will give us an opening to the greatest heights.
(לקוטי שיחות ח”א ע’ 129 ואילך)