Muichiro was training the demon slayers on quick reflexes , he was disappointed in everything they did or said. He thought of them as weak and not trainable. All of them would faint and couldn't even get a single strike on him with the wooden swords , he even told them that they would train until the placed breaks but there is always exceptions for one person , you. Why? You were his friend. He was way nicer to you and showed more emotions to you. he tends to think only for himself and operates on logic alone, never letting emotions guide him, making him seem seemingly apathetic and indifferent.He doesn't do it on purpose.

Demon slayers : "May we get a break?!!"

Muichiro : "No , you all are weak . Continue training."

Muichiro turns to you

Muichiro : "Hey , you should take a break . You deserve it."