Musar zu Schemot 12:15

The first comment on that verse, which suggests that one must pour cold water on the dough the moment it displays signs of rising, teaches the same lesson. The evil urge "heats" up a person, arouses him to commit some act in line with his "natural" desires. The commentator suggests that in order to control the evil urge one should "take a cold shower;" he uses the מצה mentioned in our verse as an illustration for the advice of how to deal with the evil urge. The commentators in the Mishnah (Pesachim 1,1) have described the search for leavened things as being conducted אור לארבעה עשר, in the night of the thirteenth to the fourteenth of Nissan. The reason is that for thirteen [years Ed.] the evil urge has uncontested rule within us. We only become equipped with the "good urge" when we officially become בר מצוה. This is why the impurity that we have accumulated within us during the first thirteen years of our lives has to be searched out and neutralised at that time. This action on our part enables the fourteenth of the month to be described in the Torah as "the first day," although on the face of it, the first day of Passover is really the fifteenth of the month. The Torah (12,15) writes: אך ביום הראשון תשביתו שאור מבתיכם "But on the first day you must destroy all leavened things from your houses, etc." This day, allegorically speaking, is the first day that we are equipped with a יצר טוב.