Webduring the hashira training arc, a bird brings zenitsu a letter explaining what happened, and shortly after we see him being all serious.
Though if i remember correctly it's not.
Webthis change stemmed from a devastating revelation delivered by his sparrow, chuntaro.
The letter conveyed the news that zenitsu’s former master, jigoro.
Webzenitsu received a letter that revealed his master jigoro kuwajima committed suicide after his apprentice kaigaku became a demon.
Webaccording to the letter, jigoro committed seppuku to take responsibility for kaigaku, who chose to become a demon and join the ranks of the upper demon moon.
Webzenitsu receives a letter from chuntaro informing him that his master, the former thunder hashira jigoro kuwajima, committed seppuku to atone for kaigaku's.
Webin this letter is a notice that zenitsu’s instructor, jigoro kuwajima, has killed himself through seppuku to atone for the acts of one of his students.
Seppuku is a japanese ritualistic suicide.
Webas revealed in demon slayer chapter 144, titled successors, zenitsu’s letter brought him news about the sudden death of his master and former thunder hashira, jigoro.
Webzenitsu received a letter that made him serious and brave in episode 7 of season 4.
The letter revealed the tragic fate of his master, jigoro kuwajima, who died.