The words חמץ and מצה with how the letters are shaped hold the secret why we eat מצה on passover.

The difference of letters in the words ח-מץ and מצ-ה is the letter ח and ה.
When the same piece of dough is in a simple flat form without anything extra it is a מצה with a ה. The letter ה has a little break on its left leg on the top which shows the openness of willing to hear from others. The person is in a receiving state like a child's mind (just like the Jews were when they left Egypt as a child going into the wilderness) without any previous notions.
But חמץ is a dough that has risen showing an arrogant person. The ח is closed on the top showing it has closed its receiving mind and is only open at the bottom willing to give ideas and not accept.
On the night of pesach, we eat the simple מצה to relive and reconnect to the spiritual state the Jews were at on this same night as a child with wonder leaving his mind open to experience anything that comes up. Like the prophet Jeremiah said