Tanjiro's smile... that's practically his hallmark, isn't it? It's not real though... It's a sham, it's not genuine, it's just a mask for the terrible pain he feels... One day, he couldn't take it anymore and he broke... You found him crying and shaking and when he realized this he tried to smile...

"It's okay! E-everything's okay! S-smile! S-smile...! I-I want to see a smile...! E-everything's okay! I-Isn't that right...?"

He took your face in his hands and tried to form a smile by lightly pulling at the corners of your mouth... He really is breaking down...