Rambam – Hilchot Deot: The Middle Path and Simcha
Mishneh Torah, Human Dispositions 2:7
The Rambam codifies that a person should not be given to excessive sadness or excessive frivolity, but should serve God with a calm and joyful disposition. The Maor VeShemesh contrasts this with the higher Chassidic ideal where joy is not merely moderate but an intense, transformative spiritual force.
לֹא יְהֵא אָדָם בַּעַל שְׂחוֹק וּמַהֲתַלּוֹת וְלֹא עָצֵב וְאוֹנֵן אֶלָּא שָׂמֵחַ. כָּךְ אָמְרוּ חֲכָמִים שְׂחוֹק וְקַלּוּת רֹאשׁ מַרְגִּילִין אֶת הָאָדָם לְעֶרְוָה. וְצִוּוּ שֶׁלֹּא יְהֵא אָדָם פָּרוּץ בִּצְחוֹק וְלֹא עָצֵב וּמִתְאַבֵּל אֶלָּא מְקַבֵּל אֶת כָּל הָאָדָם בְּסֵבֶר פָּנִים יָפוֹת.
One should neither be constantly laughing and a jester, nor sad and depressed, but happy. Our Sages declared: "Jesting and lightheadedness accustom one to lewdness." They also directed that a man should not laugh without control, nor be sad and mournful, but receive everyone in a friendly manner.