Refining the joy of Sukkos

והיית אך שמח.

"And you shall be only joyful."

The Torah commands a simchah on Sukkos, but adds the limiting word "ach" (only/but), hinting that this joy must be carefully guarded.

לאשר כי השמחה יכול להתפשט לבוא לידי פריקות עול ח"ו לכן נאמר בו אך לשון מיעוט.

Because joy can spread and lead to throwing off the yoke, chas v'shalom, therefore the word "ach" is used, a term of limitation.

Unrestrained simchah can deteriorate into pricas ol (casting off the yoke of mitzvos), so the Torah qualifies the command with "ach" to teach that joy needs boundaries.

ואחז"ל אך את הזהב שצריכין להעביר החלודה כמו כן אך שמח להיות השמחה רק לשם שמים בלי פסולת.

And Chazal said: "ach (only) the gold" — that one must remove the rust; so too "ach samei'ach" (only be joyful) means the joy should be purely l'shem Shomayim, without dross.

Just as gold must be refined of its rust, so the word "ach" demands that our simchah be purified — directed solely toward Heaven, free of any selfish "pesoles" (impurity).

ואמת אך למעט האומות כמ"ש בשמחתו לא יתערב זר ועמ"ש לקמן:

And in truth, "ach" comes to exclude the nations, as it is said: "In his joy no stranger shall share" (Mishlei 14:10) — and see what is written further on.

The word "ach" also separates the unique simchah of Bnei Yisrael from the umos ha'olam (the nations); this is a holy joy that no outsider can enter into.

Summary: The limiting word "ach" in the mitzvah of simchah teaches that the joy of Sukkos must be refined like gold of its rust — directed purely l'shem Shomayim and uniquely belonging to Bnei Yisrael, never deteriorating into a casting off of the yoke.