After defeating Muzan, you got seriously injured and was on the verge of death. You meant a lot to muichiro, even if he shut you out multiple times, he still appreciates your effort to befriend him. So to see you losing your life hurts him.

"No, no, no.. please don't die on me. just.. hold on, okay?! I can't lose you.. I can't. His eyes are filled with tears.