Purim rectifies naaseh through joy

קימו וקבלו כו' להיות עושים כו' וכתיב לעשות אותם ימי משתה ושמחה כו'.

"They confirmed and accepted... to be observing..." (Esther 9:27), and it is written "to make them days of feasting and gladness..." (Esther 9:22).

The Sefas Emes notes the language of the Megillah: Bnei Yisrael "confirmed and accepted" the observance of Purim as days of mishteh (feasting) and simchah (joy).

ומה זה שבח הגדול שקיבלו עליהם מעשה זו.

And what is the great praise in their having accepted upon themselves this practice?

He asks why accepting a holiday of feasting should be considered such a great merit for Bnei Yisrael.

אבל הענין הוא עפ"י מ"ש חז"ל הדר קבלוה בימי אחשורוש.

But the matter is based on what Chazal said: "they re-accepted it [the Torah] in the days of Achashverosh" (Shabbos 88a).

The Gemara teaches that in the time of Achashverosh, Bnei Yisrael re-accepted the Torah anew.

פרש"י מאהבת הנס כו'.

Rashi explains: out of love for the miracle.

According to Rashi, this re-acceptance flowed from their love inspired by the miracle of Purim — a willing acceptance, not a coerced one.

ותוס' מקשים הא מקודם הקדימו נעשה לנשמע.

And Tosafos asks: but beforehand they had already placed "naaseh" before "nishma."

Tosafos challenges: at Har Sinai they had already accepted the Torah willingly, declaring "naaseh" before "nishma" — so what was lacking that required re-acceptance?

ולמה הוצרך לכוף עליהם הר כגיגית.

And why was it necessary to suspend the mountain over them like a barrel [forcing them to accept]?

If they accepted willingly, why does the Gemara also say Hashem held the mountain over them, compelling acceptance? This is Tosafos' difficulty.

אבל באמת המגיד מראשית אחרית הוא ראה וידע כי לא ישארו בכתריהם.

But in truth, "He who tells the end from the beginning" (Yeshayah 46:10) saw and knew that they would not remain in their crowns.

Hashem, who foresees the end, knew that Bnei Yisrael would not retain the "crowns" they received at Sinai for their "naaseh" — they would fall through sin.

כאשר חכמים הגידו קלקלתם נעשה הזהרו בנשמע.

As the Sages said: they corrupted [the level of] "naaseh"; [therefore] be careful with "nishma."

Because Bnei Yisrael damaged the level of "naaseh" through their sins, the safeguard of "nishma" — the bound, obligatory acceptance — was needed, which is why the mountain was held over them.

וע"י שכפה עליהם הועיל אח"כ לקבל השמיעה.

And because He compelled them, it was effective afterward in receiving the "hearing."

The element of compulsion at Sinai ensured that even after they damaged "naaseh," the binding obligation of "nishma" would hold them fast.

גם שקלקלו הנעשה בחטא הידוע.

even though they corrupted the "naaseh" through the well-known sin.

Despite their having spoiled the willing "naaseh" through the cheit ha'eigel, the compelled "nishma" remained intact to anchor them.

מול זאת אמרו הדר קבלוה בימי אחשורוש נראה שניתקן עתה בימי הפורים קלקול הנעשה.

Corresponding to this, they said "they re-accepted it in the days of Achashverosh" — it appears that now, in the days of Purim, the corruption of the "naaseh" was rectified.

The re-acceptance of Purim, done out of love, repaired the damaged level of "naaseh" — restoring the willing, loving observance that had been lost.

לכן הימים נזכרים ונעשים.

Therefore "these days are remembered and observed (na'asim)" (Esther 9:28).

The Megillah's word "na'asim" (observed/done) echoes the rectified "naaseh" — Purim is precisely the repair of the level of doing.

ולכן המצוה לענג הגוף.

And therefore the mitzvah is to give pleasure to the body.

Because Purim rectifies the level of "naaseh" — the realm of physical action — its mitzvos involve bodily delight: feasting and joy.

ואיתא בזוה"ק כי בפורים מתקנים ע"י עונג כמו ביוה"כ ע"י עינוי ע"ש.

And it is taught in the Zohar HaKadosh that on Purim one effects rectification through pleasure (oneg), just as on Yom Kippur through affliction (innui); see there.

The Zohar pairs Purim and Yom Kippur (Yom Ki-Purim): both achieve tikkun, but Yom Kippur through self-affliction and Purim through physical pleasure.

וכמו ביוה"כ נגמר הכפרה ואמר סלחתי.

And just as on Yom Kippur the atonement is completed and [Hashem] said "I have forgiven" (Bamidbar 14:20),

Yom Kippur brings the kapparah (atonement) to completion, sealed with Hashem's declaration of forgiveness.

והוא ע"י תשובה במ' יום שע"ז נתקן ימי אלול לתשובה מיראה.

and this is through teshuvah over forty days, for which the days of Elul were established for teshuvah out of yirah (awe).

The atonement of Yom Kippur is reached through the forty days of teshuvah from Elul through Yom Kippur — a teshuvah driven by fear and awe.

וכמו כן בחדש אדר מאהבה ושמחה זוכין לתשובה וע"י שנתקן העשי' לכן המצוה במשתה ושמחה:

And likewise in the month of Adar, through love and joy one merits teshuvah; and because the "doing" was rectified, therefore the mitzvah is through feasting and gladness.

Just as Elul brings teshuvah through yirah, Adar brings teshuvah through ahavah and simchah. Since Purim rectifies the level of action ("naaseh"), its avodah of return is expressed specifically through mishteh and joy.

Summary: The Megillah praises Bnei Yisrael for accepting Purim, because in the days of Achashverosh they re-accepted the Torah out of love (Rashi). Tosafos asks why this was needed after the willing "naaseh" at Sinai. The answer: Hashem foresaw they would damage their "naaseh" through sin, so He also compelled "nishma" as a binding safeguard. Purim — re-accepted out of love — repaired the corrupted "naaseh," hence the days are "na'asim" (observed/done) and the mitzvah is bodily pleasure. Like Yom Kippur (Yom Ki-Purim), which atones through affliction and teshuvah of yirah in Elul, Purim atones through pleasure and teshuvah of ahavah and simchah in Adar, restoring the level of action through feasting and joy.