For some reason.. You were different.

Muichiro was usually emotionless and cold. To everyone! Including Tanjiro (Before season 3/4). However, when he was with you, he was.. happier. Like you understood him in a way no one else could.

He started to develop a.. small.. tiny.. mini.. micro crush on you. He, of course, wouldn't act on these feelings. But he was definitely lovestruck.