Introduction

The Hebrew Bible and Rabbinical literature contain numerous references to Joy in connection with complete devotion and sincere subjection of our free will and conduct to Avodat Hashem (divine worship of G-d and observance of the Torah Mitzvot or commandments).

Maimonides

Rambam (Maimonides or Rabbi Moshe Ben Maimon) DOES MENTION JOY SPECIFICALLY IN CONNECTION WITH SUCCOTH, chapter 8, Halakha 15, as quoted by Rabbi Yoel Liberman (yeshiva.org).
He also quotes sources that state that a person may even make a fool of himself in the process of over-joy for the sake of the Torah. For example, King David mightily danced in front of the Holy Ark and Sefer Torah, upon their return to Jerusalem (Samuel II 6:14), as he appeared to make a fool of himself (so to speak).

For not being happy in good heart when keeping the Torah commandments, the Torah prescribes curses and punishments, as quoted in Devarim Deuteronomy 28:45-47:

Raneinu Bechaye Ibn Pekuda

Raneinu Bechaye Ibn Pekuda [RBIP], a giant rabbinical scholar and philosopher, 11th century, Spain, writes in his monumental magnum opus book - Duties of the Heart (Chovot Halevavot), source: Aspklaria, an online Jewish Alphabetical reference):
A person should keep away from pure physical joy that leads to over-indulgence in physical pleasure which could lead to sin, and he should not rejoice in the misfortune of others, or be depressed.
The latter could be detrimental to his health as melancholy could dry his bones (see Mishlei Proverbs 17:5 & 22). According to RBIP; the joy involved in fulfilling a Mitzva (Torah commandment) brings about a reward GREATER than the reward for fulfilling the Mitzva itself! This is truly amazing.

Shulchan Arukh

While Shulchan Arukh (codified Jewish law) does not mention being happy as a specific positive Torah commandment, indirectly it does so, because being happy contributes to good health, as mentioned above.

Our tradition of Simcha

According to the sages, in particular Chassidic teaching; excessive rejoicing on Purim and Succoth, for the sake of the Torah, has an immense power to atone for and convert our transgressions to merits.
Simchat Torah (rejoicing with the Torah) at the end of Succoth is a uniquely momentous occasion, which we should all grasp, to merit another wonderful year in health, wealth and happiness, and hasten our final redemption.
Such moment of true divine joy should remain with us for the rest of the year.

Selective sources about being joyful
Devarim Deuteronomy 12:7-12, Psalms 2:11, 35:27, 97:11-12, 100:2, and 113:9.