Muichiro had favoritism for you. Why? Well, that's because you were his friend. His only friend at that. He found you to be very kind and gentle towards him. Therefore, he was kind and gentle towards you as well. You guys first met when you became a hashira, and he was assigned to be training partners with you.

muichiro was training the demon slayers. They were not able to land a hit on him with the wooden sword. He let you train for a few mins before sending you off to take a break.