Translated by Ralph Anzarouth and an anonymous friend
According to some opinions, joy is a consequence of love. It consists in serenity and satisfaction about a particular topic and it is beneficial to man's health. King Solomon said (Proverbs 17, 22): "A happy heart will benefit one's body and a depressed spirit will stiffen one's limbs". However, one should make use of this trait with morality and proper conduct and not enjoy his fellow man's misfortune, as it is written (Proverbs 24, 17): "Do not rejoice in your enemy’s downfall, and do not let your heart exult in his failure", and he should not become frivolous, as our Sages of blessed memory said (Ethics of the Fathers 3, 13): "Frivolity and lightheadedness bring man to immorality".
Man should only rejoice in his lot since in that way he will be truly rich, as it is written (ibid 4, 1): "Who is the rich man? He who rejoices in his lot.". He should also delight in the Lord, as it is written (Psalms 32, 11): "Delight in Hashem and the just will celebrate and all the righteous will exult". It is further written (ibid. 105, 4): "The heart of those who seek Hashem will rejoice". It is also a major stepping stone to attaining the Spirit of Holiness, as described by our Sages of blessed memory (Talmud Bavli, tractate Shabbat 30b): "Prophecy only habits a person who is in a state of happiness". They further said (ibid.): "[The verse] (Eccl.15, 8) 'I praise joy' applies to rejoicing over the performance of a Mitzvah". And (Psalms 14, 7) "Jacob will delight and Israel will rejoice" is in everything pertaining to holiness.
Concerning this trait in connection with carrying out Mitzvot, our Sages of blessed memory cite numerous events and in particular Simchat Beit Hashoevah1, which they used to celebrate in the Temple [of Jerusalem] during the Sukkot festival, about which it is said that one who has never seen this event has never seen true rejoicing in his life. It is written in the Talmud Bavli (tractate Sukkah, 53a): "They said about Rabban Shimon ben Gamliel that when he celebrated the Simchat Beit Hashoevah he used to juggle with eight flaming torches etc., a feat that no human could ever perform etc.", just as King David danced and frolicked in front of the Holy Ark2 and said (Second Book of Samuel 6, 22): "I would be ready to be humiliated even more than this3 etc.".
Similarly, our Sages of blessed memory instruct us to make a bride and groom happy. It is written in Isaiah (25, 9): "This is our Lord in Whom we trust, we shall rejoice and delight in His salvation".
Notes of the translators:
 Simchat Beit Hashoevah was a ceremony which consisted in drawing water to be poured on the Altar only during Sukkot. This was carried out through a joyous popular procession culminating with a great celebration in the Holy Temple of Jerusalem.
 When the Ark had been recovered from enemy hands and was being brought to Jerusalem.
 In order to express his joy. This was King David’s answer to criticism regarding his apparently unseemly behavior at the aforementioned event.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 8 about joy is found at pages 17-18.