(Parashat Vaetchanan contains the first paragraph of the Shema prayer, in which the mitzvos of both tefilin and mezuzah are found.)

Rabbi Hiya said, whoever wishes to observe his ways [and not be injured] must not pass waste water before the door, because a certain demon lies there between the two doorposts facing the door. It sees whatever is done in the house. One must not spill water between the two doorposts. Rabbi Yitzhak said, Clean water is not forbidden [to spill between the two doorposts], but it must not be spilled with contempt [which would nourish the external forces]. What is the reason? It is because [that demon] has permission to cause harm. Moreover, it turns its head towards the house and whatever it beholds becomes cursed.
...a certain demon lies there between the two doorposts facing the door.
It has 365 servants, as the number of the days in the year, and it rules over them. They all go out with a person when he goes out the door of his house. Rabbi Elazar said, all this transpires because G‑d wished to guard Israel [from the external forces] and established His Holy Name above, which is the Torah (written in the Mezuzah for our protection). The whole Torah is one Holy Name, and whoever is occupied with the Torah is occupied with His Name.

Come and see, man should imprint the Holy Name [in the two paragraphs of the Mezuzah, for the Torah is composed of Names of G‑d], which is the universal faith on the door of his house, for wherever the Holy Name abides there are no types of evil, and they cannot bring charges against him, as written, "No evil shall befall you." (Psalms 91:10)

The place where the door of the house lies is a reflection of the higher. The place where the door of the supernal house rests is called "Mezuzah" [the opening of the door of the World of Atzilut, is the gate for all supernal crowns, the gate for all who connect to the Holy Name], which brings the house and the door of the house to a state of completion [as well as to the house of Atzilut].
...when a man affixes a Mezuzah at the door of his house...that man is adorned with his Master's crowns...
Agents of the law [the external forces] flee that Mezuzah, and the prosecutors cannot dwell before it. Correspondingly below, when a man affixes a Mezuzah at the door of his house, and this Holy Name is imprinted with its letters [and the Holy Name Sha-dai], that man is adorned with his Master's crowns and the types of evil do not come near the door of his house.

BeRahamim LeHayyim:
A mindful life takes the mundane and finds sacred connection. Like the simple door jam which links two different environments both within and without. Inside we have the safe private place of unity with those we are either related to or have hung out together because of some common purpose or liking. Outside is the place of diversity, unpredictable and chaotic. Between the two, we Jews are commanded to write passages of Holy words to Safeguard the Doors of Israel/Shomer Daletot Yisrael--whose initial letters of Shin Dalet Yud stand for the Divine Name Sha-DaY, praised for its protective powers. This mezuzah is placed on the left side of the door, the place of restriction, sweetened by the parchment with the sacred writing.

As we make the transition from safe to precarious, we are to hesitate briefly and make the moment special, and in fact holy, the source of the custom to touch one's finger tip to the Mezuzah and to kiss it.

Live in the moment when you leave your homes, as well as all day, until you return to the Mezuzah of your welcoming shelter.

Bracketed annotations from Metok Midevash and Sulam commentaries
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