Moussar sur Le Deutéronome 28:47

היום הזה ה' אלוקיך מצוך . Rashi explains that the commandments should appear to us as if they had only been made on that very day. Rashi adopts the same approach when explaining in 27,9: "Keep silence, and hear O Israel, this day you have become a people to the Lord your G–d." Rashi writes: "You should consider each day as the one on which you entered into the covenant with G–d." This is a very important rule, which, when practiced, helps us to perform G–d's commandments with eagerness, as one performs a task newly assigned. It helps not to treat Torah as something that we are so familiar with that we fall into the habit of neglecting its demands. The Arizal explains 28,47: "because you did not serve the Lord your G–d in joy and with a glad heart when you enjoyed everything in abundance," in a similar vein. G–d teaches us that performance of His commandments must be accompanied by a greater joy than the joy one feels for all the material blessings G–d has bestowed on one. It is not enough merely to serve the Lord and obey His commandments. We must do so joyfully.

Secondly - because on other holidays, we establish a meal for the joy of the commandment. For the reward for joy over the commandment increases and grows, as it is stated (I Chronicles 29:17), “now Your people, who are present here - I saw them joyously making freewill offerings.” And it is stated (Deuteronomy 28:47), “Because you would not serve the Lord your God in joy and gladness.” But since the fast is on Yom Kippur, they were obligated to fix the meal over the joy of the commandment, on the eve of Yom Kippur.

Even on Holidays and Festivals, of which it is written : "And you shall rejoice before the Lord your God" (Deut. 12:18), one should not prolong his drinking more than is proper, as it is said : "Because you did not serve the Lord your God with joyfulness and with gladness of heart" (Ibid. 28:47). From this you learn that we have not been commanded to rejoice in a manner that the Creator of all is forgotten. For it is impossible to serve the Lord, may He be Blessed, out of light headedness, or laughter, or drunkenness.