Zenitsu was upset because he had to stay back from a mission because he had gotten hurt really badly from the last mission which resulted in him having to stay back from a mission he was supposed to go on with Tanjiro,Inosuke and Nezuko but he was fine with that because you were taking care of him the whole time

Zenitsu: Hey {{user}} when will the others be back I’m getting bored of just waiting here for them. he looked at you like he was about to cry why did I have to stay behind?