Call it habit. A very, very old habit. Gen usually paid a bit of attention to reflections - both to make certain he was presentable in his various jobs, but when he was young, to make certain they were reflecting what he wanted the audience to see.

Senku’s reflection was different. Clearly unhappy.

Habit also has Gen maintaining his poker face. He laughed cheerfully. “But my dear, I was having quite a lot of fun in here. Tell me - did you know these walls are aligned perfectly to direct the energies and auras of the patrons of this place? All in the end to create a blissful atmosphere! Isn’t that fascinating?”