Why do I get so irritated when my friend doesn't do her homework

Ok bare with me because I also dislike this trait about myself but whenever we have homework due in for school, my best friend never does it, or its extremely half-arsed meanwhile I've started putting in more effort (We're both at A-Level so the homeworks take 2-3 hours respectively for each subject). I get the most frustrated when he gets away with it because some of our teachers don't really check it/ forget to and it feels like my hard work is being completely undermined, not to mention how he has never once gotten detention for not doing it (always manages to get away with it). I recognize that in the long run, its beneficial for me to do it but I still can't help feeling really agitated about it, how on earth do i stop caring about what he's doing/ not doing and focus on myself? Am I a terrible friend for wanting him to get in trouble?