It’s a joke that’s tied to his character, but why did Rengoku keep saying delicious?
Rengoku chose to keep saying delicious because he was eating a bento box. Tanjiro, Inosuke, and Zenitsu board the Mugen Train and hear Rengoku enjoying his food.
An anime-exclusive filler episode details how Rengoku acquires his bento box and why it means so much to him.
Rengoku eats his food, keeps saying delicious
Chapter #54 begins with Tanjro, Zenitsu, and Inosuke preparing to board the Mugen Train. Tanjiro wants to board the Mugen Train to meet Rengoku, a Flame Hashira who can potentially shed some light on the Hinokami Kagura technique.
Inosuke, being raised in the wilderness, is terrified of the train. He calls it the “lord of the land,” and marvels at its ability to slide across the Earth.
Tanjiro, who also lives a sheltered life, assumes the train to be the guardian deity of the station. Zenitsu explains to the group what a steam train actually is.
The group prepares to board the train. Before doing so, Inosuke draws attention to himself by head-butting the side of the vehicle. The local police, noticing the group’s swords, give chase. They’re able to get away and properly hide their weapons before boarding the train properly.
Zenitsu explains how the Demon Slayer Corps is not officially recognized by the government. That means that Demon Slayers can not openly carry their weapons in public areas without causing fear and scrutiny.
Within the train, Inosuke shouts out “We’re in its belly! Grahh! Let’s take it down!” Zenitsu asks Tanjiro how Rengoku can be identified, with his distinctive hair and scent being the answer.
Suddenly, the group hears the word “delicious” shouted over and over again. They approach the back of the train car to see Rengoku chowing down on a bento box. He’s repeatedly yelling “delicious” with each new bite he takes. This is the first time that Tanjiro meets Rengoku. The Demon Slayer keeps this bright and infectious smile on his face when he passes away, as well.
So that’s the answer. Rengoku chose to keep saying delicious because he was eating a bento box when Tanjiro, Inosuke, and Zenitsu arrived.