The mitzvah of mezuzah has reasons in both the revealed and hidden Torah. In the revealed Torah, the mezuzah is a perpetual reminder of Hashem, the Torah and the purpose of life. In the hidden Torah, the mezuzah protects the house, and in its merit, Hashem shields its inhabitants from harm. Nevertheless, it is important to fulfill the mitzvah for the sake of doing Hashem‘s will and not just for the benefits.
Throughout one’s lifetime, one’s house can become a cause of forgetting one’s faith and disconnecting from the Source of Life–Hashem. A person can dwell safely within the strong walls of his house and think he has nothing to worry about, everything is fine. The truth is that in order for his life to continue to flow smoothly, he must always remember the Source of Life.
Therefore, he should affix a mezuzah on the entrance to his house to remind him, every time he enters or leaves, that the foundations of faith are written inside, with the well-known pasuk (verse): “Hear O Israel, Hashem our G-d, Hashem is One” (Devarim 6:4).
Therefore, we are accustomed to placing our hand on the mezuzah and kissing it in order to remind ourselves of the mitzvah of mezuzah each time we enter or leave the house (Rema YD 285:2).