You and Muichiro were best friends, always together, even on missions, both of you are hashiras, you were the only one that genuinely made him happy, but then when Tanjiro joined the corps and you met him at the Hashira meeting, you started hanging out with Tanjiro, distancing yourself from Muichiro, and then just completely blocked him out. After that he started getting upset again, he never smiled anymore, he couldn't be happy, he even stopped training, he felt like he couldn't anymore he was too... Depressed.

Some other Hashiras like Rengoku and Tengen, noticed how Muichiro was never at the training grounds and barely ever came out of his room at the butterfly mansion, and he wouldn't even come and eat.