The Gemara in Masechet Yoma (4b) tells that the Torah was given to Benei Yisrael at Sinai with "awe and fear, trembling and dread." As the Gemara explains, this sense of fear that accompanied Matan Torah (the giving of the Torah) is an expression of the verse in Tehillim (2:11), "Gilu Bi'r'ada" – "Rejoice in fear" – which requires that even when we rejoice in the Torah, we must do so with a sense of awe and reverence. Therefore, even though Matan Torah was an exceptionally joyous occasion for Am Yisrael, it had to be accompanied by a sense of dread and fear.
This required balance between joy and fear in the performance of Mitzvot finds expression in several Halachot.
1) The Rashba (Rabbi Shlomo Ben Aderet, Spain, 1235-1310), in one of his responsa (1:215; listen to audio for precise citation), addresses the case of a Chazan who has a beautiful voice and prolongs the prayer service so that the congregation will hear and enjoy his melodious singing. The Rashba writes that if the Chazan stands with a sense of awe and fear as he conducts the service, then he has indeed fulfilled the dictum of "Gilu Bi'r'ada," combining the joy of prayer with the required aura of reverence. If, however, his intention is solely that the congregation hears his voice and showers with him praise, then he acts improperly. Such a Chazan conducts the service only with the element of joy, but without the required sense of reverence.
2) There is a widespread custom that a groom, upon the completion of the wedding ceremony, breaks a glass. The Rokei'ach (Rabbi Elazar of Worms, Germany, 1160-1237) explains this practice (in Siman 353) as intended to ensure this balance between joy and fear. The breaking of a utensil is meant to temper somewhat the joy of the groom and the guests, who might otherwise become so involved in their celebration that they lose sight of the importance of maintaining a sense of fear and reverence.
3) Chacham Ovadia Yosef (in Yabia Omer, vol. 7, Orach Chayim 56) discusses the practice of some people on Simchat Torah and during a Sefer Torah dedication ceremony to toss the Sefer Torah in the air and then catch it. Chacham Ovadia denounces this practice, ruling that it is most certainly forbidden. Besides infringing upon the respect due to a Sefer Torah, it also bespeaks an absence of fear and trepidation while engaging in the Mitzva. While these are certainly very joyous occasions involving a Mitzva, the celebration must be tempered by a sense of awe and fear.
Summary: The required balance between joy and fear in the performance of Mitzvot finds expression in several contexts in Halacha: 1) A Chazan must conduct the service not merely with the intent that the congregation should hear and enjoy his pleasant voice, but also with a sense of awe and dread. 2) A groom breaks a glass after the wedding ceremony in order to temper the festive celebration. 3) When one dances with a Sefer Torah on Simchat Torah or during a Sefer Torah dedication, he may not toss it up in the air as part of the celebration.
