Joy Completes Every Mitzvah
בפסוק והי' אם שמוע תשמעו כו' אנכי מצוה אתכם היום הה"ד אשרי א' שומע לי לשקוד על דלתותי הם השערים הנפתחים בכל יום ע"י המצות שכל מצוה היא פתח ושער לעבודת השי"ת
On the pasuk "And it will be, if you surely listen... that I command you this day" (Devarim 11:13), this is the meaning of what is written: "Fortunate is the man who listens to Me, to keep watch at My doorways" (Mishlei 8:34) — these are the gates that are opened anew each day through the mitzvos, for every mitzvah is an opening and a gate to the avodah of Hashem.
The Sfas Emes connects the parshah's opening words about listening to Hashem's commands with the pasuk in Mishlei about watching at Hashem's gates. Each mitzvah is itself a fresh gateway into serving Hashem that opens up anew every single day.
לשקוד ע"ד רמ"ח מ"ע
"To keep watch at My doorways" — these are the two hundred and forty-eight positive mitzvos.
The phrase about keeping watch at the doorways alludes to the two hundred and forty-eight positive mitzvos that a person must actively perform.
לשמור מזוזות פתחי שס"ה ל"ת
"To guard the doorposts of My entrances" (Mishlei 8:34) — these correspond to the three hundred and sixty-five negative commandments.
The parallel phrase about guarding the doorposts alludes to the three hundred and sixty-five negative commandments that a person must guard himself against transgressing.
והי' לשון שמחה כפי השמחה והשתוקקות האדם אל המצות כך זוכה לשמוע ולקיים
"And it will be" (vehayah) is an expression of joy, for in accordance with a person's joy and yearning for the mitzvos, so does he merit to hear and to fulfill them.
The word vehayah signals joy, teaching that one's capacity to truly hear and keep the mitzvos is measured by how much joy and longing he brings to them.
והי' א"ש בשמחה
"And it will be" — to listen with joy.
Reading the verse this way, vehayah instructs a person to approach his listening to Hashem with simchah.
עי"ז תשמעו
Through this joy, "you will surely listen."
It is specifically by means of that joy that a person comes to genuinely listen and fulfill what Hashem commands.
וכן איתא ששכר מצוה מצוה פי' הברטנורה הנאת המצוה מצוה
And so it is taught that "the reward of a mitzvah is a mitzvah" (Avos 4:2), which the Bartenura explains to mean that the enjoyment of the mitzvah is itself the mitzvah.
The Sfas Emes brings the teaching that the reward of a mitzvah is a mitzvah, citing the Bartenura that the very pleasure one takes in a mitzvah is part of the mitzvah itself.
והיינו שע"י הנאה והשמחה מקיים המצוה בשלימות וזה הכנת המצוה לכן תיקנו ברכה קודם המצוה:
That is to say, through the enjoyment and joy a person fulfills the mitzvah in its completeness, and this is the proper preparation for the mitzvah — and therefore Chazal instituted a berachah before the mitzvah.
Joy in a mitzvah is not incidental but the preparation that brings the mitzvah to completeness, which is why Chazal established that we recite a berachah beforehand to arouse that joy.
Summary: The Sfas Emes reads the parshah's call to listen to Hashem together with the pasuk in Mishlei describing one who watches at Hashem's gates and doorposts, explaining that every mitzvah is a fresh gateway into the avodah of Hashem that opens anew each day — the doorways alluding to the two hundred and forty-eight positive mitzvos and the doorposts to the three hundred and sixty-five negative commandments. He teaches that the word vehayah is an expression of simchah, for a person's ability to truly hear and fulfill the mitzvos depends on the joy and yearning he brings to them. This is why "you will surely listen" follows from the joy of vehayah. He strengthens this with the teaching that the reward of a mitzvah is a mitzvah, which the Bartenura understands to mean that the enjoyment of a mitzvah is itself part of the mitzvah. Joy is thus the preparation that brings a mitzvah to its completeness, which is why Chazal instituted reciting a berachah before performing it.