i didn’t know what the song was named, so I literally only realized today that Sia is saying “I want to swing from the chandelier" and not, like, chantilly ear or something
it’s not very optimistic, but my personal theory is that Sousuke has known about his injury for a while, and it’s serious enough that there’s a good chance he won’t have a full competitive swimming career ahead of him.
Sousuke comes across as a smart guy, so it seems idiotic to me that he’d still be competing with his injury as bad as it seems to be. at least based off of my minimal knowledge of sports medicine type stuff, continuing to stress an injury instead of giving it adequate time to heal is kind of the pinnacle of bad ideas. and Sousuke has to know that. which is why I suspect his injury isn’t something that a few months of rehab will fix and then he’ll be good to go for years of high intensity swimming.
Sousuke clearly doesn’t want Rin to lose his swimming dream. if Sousuke’s competitive future is tenuous at best, I can see how he could have the rational to throw that slim chance away in the hopes of kicking Rin’s ass into gear.
and then if you add in Sousuke’s out-of-character desire to be a part of the relay: I think he’s desperately trying to experience the friendship-power Haru and the gang have. right now his only reason for swimming is to be the best. and if his ability to swim that well gets taken away from him, he doesn’t have a reason anymore.
(and, possibly, if your swimming career is going to prematurely end anyways, why not go out in a blaze helping your friend win instead of slowly petering out?)