“And the sons of Kahas, he (Moshe) did not give (wagons), because the holy service was upon them...” (Bamidbar, 7:9)
The other Levite families were given wagons to carry the sections of the Tabernacle during travel; but the Kahas family was commanded to carry the holiest vessels (the holy Ark, the Table, etc.) on their shoulders. As the verse continues, “ba’kosef yi’sau’u” - “on the shoulders, they carried.
”The Talmud (Archin, 11a) questions the inclusion of the word ”yi’sau’u” - “they carried.” Would it not have been enough to write, “because the holy service had to be on the shoulders”? It would be understood that they carried it. The Talmud concludes that the word “yi’sau’u” must be hinting at an obligation to sing. (As in “s’ue zimrah” - lift up (your voice) in song. - T'hillim, 81:3).
That singing is hinted in such a way, leads to the deduction that their obligation to sing was inspired by the very labor of carrying, “(with) the shoulder-carrying, they (began, of themselves, to) sing.
”A similar phenomenon is found in Shmuel I, 6:12). When the Philistines returned the Holy Ark they had captured via oxen, the verse records “And the oxen sang on their journey.” The Zohar comments, “the Ark on their backs inspired them to sing (Zohar B’reishis 123a) - even oxen were inspired from the labor of carrying the holy Ark!
Whenever we toil to worship G-d sincerely, we discover that we have become full of happiness and song. Prayer from the heart, Torah, mitzvos - all these touch the soul, and fill our hearts with joy. (Sfas Emes, Naso 5641)
On Yom Kippur, that most solemn and awesome of all days, when our fate for the year is sealed - we pray fervently all day trying to attain true repentance. But our serious mood ends when the day ends, because the halacha (Shulchan Aruch, 625:5) is to eat and rejoice. This is based on the Midrash which says that as Yom Kippur ends, a Heavenly voice calls out “Go and eat with joy!” (Koheles Rabba, 9:7). Perhaps that 'voice' is the spirit of joy which comes in the wake of the day's mitzvos and prayers.