ויקרא ז, יא: וְזֹאת תּוֹרַת זֶבַח הַשְלָּמִים
Nothing leavened is brought on the Altar, with a few special exceptions. One of these exceptions is the ten loaves of leavened bread (out of the forty total loaves) which are brought in conjunction with the thanksgiving offering (one of the categories of the Peace offering). The Sforno explains that this exception is to symbolize what Chaza”l call שְאוֹר שֶבֱעִּסָה (the leaven in the dough, see Berachos 17a). Rashi explains there that this expression refers to the evil inclination in the heart of a person.
The Thanksgiving offering is brought by someone who was delivered from peril. The very fact that a person was exposed to danger indicates a vulnerability due to spiritual shortcomings. However, HaShem in His mercy saved this person. Thus, the loaves of leavened bread symbolize this concept.
