Appropriate Joy
Shulchan Aruch (529:3) writes, "One should eat, drink, and be joyous on Yom Tov, but the focus should not be on the meat, wine, laughter, and lightheadedness, as laughter and lightheadedness do not constitute true joy. They are rather levity and foolishness. We are not obligated to engage in silliness and foolishness, but rather in the joy of serving Hashem."
When one attends a simchas beis hashoeivah in our times, he will generally see everyone dancing. But this wasn't how it was in the Beis HaMikdash. The Rambam (Hilchos Lulav 8:14) writes, "The dancing at the simchas beis hashoeivah wasn't performed by amei ha'aretz or just by anyone who wanted to dance. Only the gedolim, chachmei Yisrael, roshei yeshivos, the heads of Sanhedrin, chassidim, elders, and anshei ma'aseh were worthy of dancing. They danced, clapped their hands, played music, and were happy in the Beis HaMikdash during Succos, but the rest of the nation, men and women, came to watch and listen."
The Ritva (Bava Basra 121.) explains that the typical person wasn't permitted to dance and perform stunts to ensure that the joy remained holy and that it shouldn't turn into הוללות, levity.
Sfas Emes zt'l suggests that this concept is hinted at in the word 'אך' from the verse 'שמח אך היית'. 'אך' is a term that denotes limitation. According to this interpretation, the Torah instructs us 'שמח אך והיית', to be exceedingly joyful, but not all forms of happiness are beneficial. We must restrict the kind of joy we partake in. We should only seek the joy that originates from and leads to a connection with Hashem.
Reb Yosef Chaim Sonnenfeld zt'l said that this is based on the Chazal, לרבות אלא מיעוט אחר מיעוט אין, that whenever there is more than one מיעוט, limitation, expressed in the Torah, it means that we should increase rather than decrease. Therefore, when we sing שמח אך והיית, and we repeat the word אך to increase our joy, not to decrease it at all. We should be extremely happy on yom tov.
Or, it can be explained this way: אך is a מיעוט, implying that we must know that even a tiny amount of joy is worthwhile and great accomplishment.
Another approach is that אך is a limitation, which refers to when things aren't going well for a person. The Torah instructs us שמח אך והיית; even then, you should be happy.
