Look at this list:
- After six weekdays (worries) we have Shabbat (pleasure).
- Five days after 9 be-Av (saddest day): Tu be-Av (happiest day).
- Seven weeks after Pesach (freedom): Shavuot (Torah learning).
- Four days after the Days of Awe: Sukkoth, and Simchat Torah.
There is a trend here. Happiness at the end. Sukkoth is the time to celebrate. There is no greater joy than from learning Torah. Shabbat also leaves us extra time to learn Torah.
Jews need to do the Commandments happily. That is not an optional embellishment. It’s an obligatory integral part of the Commandments. Doing them without happiness is not imperfect – it’s a terrible sin (Deuteronomy 28:47), what we can see from the warnings and hopefully deterring punishments (Deuteronomy 28:15-69).
G-d loves us so much — He does not want the inevitable grave days distract us from our obligatory happiness, so He helps us by putting happier days last.