What is the reason we wear tefillin and affix mezuzos?

Let us begin with the words of the Talmud Bavli in Meseches Menachos 33b:

אמר רבא מצוה להניחה בטפח הסמוך לרשות הרבים. מאי טעמא? רבנן אמרי כדי שיפגע במזוזה מיד. רב חנינא מסורא אומר כי היכי דתינטריה. אמר רבי חנינא בוא וראה שלא כמדת הקב”ה מדת בשר ודם, מדת בשר ודם מלך יושב מבפנים ועם משמרים אותו מבחוץ, מדת הקב”ה אינו כן. עבדיו יושבין מבפנים והוא משמרן מבחוץ שנאמר, ה’ שומרך ה’ צלך על יד ימינך (תהילים 121:5).

Rava said, it is a mitzvah to place the mezuzah within the tefach closest to the street. What is the reason for this? The Rabbis say, “So that one should encounter the mezuzah immediately upon entering the doorway. Rav Chanina from Sura says, “So that the mezuzah should guard the house.” Rav Chanina said, “Come and see how unlike the characteristic of the Holy One, Blessed is He, is the characteristic of human beings made of flesh and blood. The characteristic of flesh and blood is that the king dwells inside his palace and the people guard him from the outside. But the Holy One, Blessed is He, is not like this. Rather, his servants dwell inside their homes and he guards them from the outside, as it is stated: Hashem is your guardian, Hashem is your shade at your right hand.” (Tehilim 121:5)

Menachos 44a (at the bottom):

אמר רב ששת כל שאינו מניח תפילין עובר בשמונה עשה… וכל שאין לו מזוזה בפתחו עובר בשני עשה וכתבתם וכתבתם ואמר ר”ל כל המניח תפילין מאריך ימים שנאמר ה’ עליהם יחיו ולכל בהן חיי רוחי ותחלימני ותחייני (ישעיה לח:טז)

Rav Shashis said, “Anyone who does not wear tefillin transgresses eight positive mitzvos…and anyone who does not have a mezuzah on his door transgresses two positive mitzvos, as it says in the Torah: and you shall write, (and it says a second time) and you shall write. Reish Lakish said anyone who wears tefillin will have their days legenthed as it says, “Lord, concerning them, (You said) they shall live, and before all of them the life of my spirit, and You cured me and gave me life.” (Isaiah 38:16)

Rambam Chapter 6 Hilchos Tefillin

חיב אדם להזהר במזוזה מפני שהיא חובת הכל תמיד וכל זמן שיכנס ויצא יפגע ביחוד השמם,  שמו של  הקדוש ברוך הוא ויזכר אהבתו ויעור משנתו ושגיתו בהבלי הזמן וידע שאין דבר העומד לעולם ולעולמי עולמים אלא ידיעת צור העולם ומיד הוא חוזר לדעתו והולך בדרכי מישרים. אמרו חכמים הראשונים כל מי שיש לו תפילין בראשו ובזרועו, וציצית בבגדו, ומזוזה בפתחו – מוחזק הוא שלא יחטא, שהרי יש מזכרין רבים והן הןzן
(המלאכים שמצילין אותו מלחטא, שנאמר חונה מלאך השם סביב ליראיו ויחלצם (תהלים לד,ח

The Rambam in Sefer Ahavah chapter six, halacha thirteen: A person is obligated to be careful in matters relating to mezuzos because it obligates all of us at all times, and whenever one enters or leaves he will connect with the concept of unity of Hashem, The Holy Blessed One, and he will remember to contemplate his love for Hashem, and he will awake from his (spiritual) slumber, and his state of carelessness in matters of the spirit, falling into the spiritual pitfalls of the times. This will lead him to know that there is nothing that is eternal except knowledge Rock Of The World. This will cause him go immediately return to the mentally sane status of a believer and he will go forth to his next activity in a straight way. The early sages said, “Anyone who has tefillin on his head and on his arm, and tzitzit on his garment, and a mezuza on his door, he has a status of ‘one who will not come to sin’, as a result of him having having many reminders. These reminders are themselves the angles that protect him from transgression, as it says in Tehilim 34:8 An angle of Hashem encamps around those who fear him, and he rescues them.

The Rambam apparently understands that when the Gemara talks about ‘encountering’ a mezuzah it means more than just passing a mezuzah and noticing its existance. The ‘encounter’ that the Gemara is talking about is the thought process that occurs when the person encounters the mezuzah. The Rambam also teaches us that the lengthening of one’s days by fulfilling the mitzvah of mezuzah is a result of the increased awareness of Hashem that the person haseach time he contemplates his tefillin and mezuzos. The mezuzah protects and gives life because it helps the person to become a better person. This was the message of Onkelos, the ger tzedek to the soldiers. I will continue soon with his story.

The Tur in simin 25 of Aurach Chaim:

אמר ר יחנן, הרוצה לקבל עול מלכות  שמים שלמה יפנה, ויטול ידיו,ויניח תפילין, ויקרא קריאת שמע ויתפלל, Rebbe Yochanan said, “one who wants to accept upon himself the yoke of the kingdom of heaven in the most complete way should wash and prepare his body, put on tefillin, recite shema, and daven.

Rabbi Yochanan teaches us that the wearing of tefillin is an integral part of the overall process of accepting upon one’s self the yoke of Heaven, being מקבל עול מלכות שמים שלימה. Tefillin is the highly symbolic crown that we wear while we undertake each day to dedicate ourselves to fulfill the tenets of this yoke. From the words of the parshios themselves, including the details of each of the letters with their proper tagim, crowns, to the letter ‘shin‘ on the bayis shel rosh, and the letters dalit and yud that are formed with the straps,which spell out Hashem’s name,  each detail expresses a different aspect of our relationship with Hashem and his torah.