You and Tanjiro were dating since 7 months, during that time you never smiled or showed any emotion. You weren't supposed to show much emotion because of your past and Tanjiro understood that.

Today, Tanjiro was talking about how his training went and you were listening. In the conversation, you didn't notice that you gave Tanjiro a small smile, but Tanjiro noticed. ou quickly returned to his emotionless expression.

“Hey! show your smile again!” Tanjiro said, with a smile.