Torah's light infuses the body

איתא בגמ' הכל מודין בעצרת דבעינן נמי לכם שהוא יום שניתנה תורה לישראל.

It is taught in the Gemara: all agree that on Atzeres (Shavuos) we require also "for you" (lachem), for it is the day on which the Torah was given to Yisrael.

The Sefas Emes cites the Gemara (Pesachim 68b) that even those who otherwise dispute hold that Shavuos must include physical enjoyment "lachem," because it commemorates the giving of the Torah.

כי הורדת התורה מן השמים הוא התפשטות אור התורה למטה.

For the bringing down of the Torah from the heavens is the spreading of the light of Torah below.

Mattan Torah was precisely a drawing of the Torah's light down from Above into the lower, physical world — a movement of revelation into the realm of the body.

וכמו כן בבנ"י בעצמם הארת הנשמה בגוף.

And likewise within Bnei Yisrael themselves — the illumination of the neshamah within the body.

The same dynamic occurs within each person: at Mattan Torah the soul's light shone into and permeated the body, the inner parallel to Torah descending into the world.

כי האדם כולל שמים וארץ.

For a person includes heaven and earth.

Man is a microcosm, comprising both heaven (soul) and earth (body), so the cosmic descent of Torah mirrors an inner event within him.

נשמ"ה גי' השמי"ם.

"Neshamah" has the numerical value of "ha'shamayim" (the heavens).

The gematria of neshamah equals that of ha'shamayim, confirming that the soul is the "heaven" within man, paralleling the heavens above.

ועיקר נשמה יתירה קיבלו בנ"י במתן תורה אנכי ה' אלקיך.

And the essential additional soul Bnei Yisrael received at Mattan Torah — "I am Hashem your God" (Shemos 20:2).

At Sinai, with the word "Anochi," Bnei Yisrael received the primary neshamah yeseirah — an added measure of soul-light infused into them through the revelation.

ולכן אז נתבטל הגוף אל הנשמה.

Therefore at that time the body became nullified to the neshamah.

With the soul's light so powerfully revealed, the body underwent bittul — subordinating itself entirely to the neshamah.

וז"ש נמי לכם שאז הלכם מתבטל אל השורש ומעין זה בכל שבת קודש נשמה יתירה שבו מתנוצץ הארה מקבלת התורה כמ"ש זכור כו' יום השבת בעצומו של יום שניתן בו התורה ולכן מצוה לענג בו גם הגוף:

And this is the meaning of "also for you" (lachem) — that then the "for you" itself becomes nullified to its root; and a semblance of this is in every Shabbos Kodesh, [through] the neshamah yeseirah within it, in which an illumination from the receiving of the Torah sparkles, as it is written, "Remember... the day of Shabbos" — on the very day on which the Torah was given; and therefore it is a mitzvah to give pleasure on it to the body as well.

"Lachem" — the physical "for you" of Shavuos — is required precisely because on that day even the bodily dimension is nullified to its root in the soul, so physical enjoyment itself becomes holy. A reflection of this occurs every Shabbos, when the neshamah yeseirah carries a spark of the original receiving of Torah (Shabbos too is linked to Mattan Torah); therefore one is commanded to delight the body as well, since on these days the body shares in the soul's elevation.

Summary: All agree Shavuos requires physical enjoyment "lachem" because it is the day the Torah was given — and the descent of Torah from heaven into the world parallels the illumination of the neshamah within the body (neshamah equaling ha'shamayim in gematria). At Sinai, through "Anochi," Bnei Yisrael received the essential neshamah yeseirah, and the body was wholly nullified to the soul, so that even the physical "lachem" returns to its root. A reflection of this recurs each Shabbos through its neshamah yeseirah, which is why it is a mitzvah to delight the body on these days as well.