look his assigned highschool trope is "shithead bully" so he feels at ease around people he can look down on, and atsumu really just did a Big Stupid, how can he not like the guy a little bit :)

that said, i do hope they can get a little bit chummy! gen would definitely be up for training atsumu a bit in combat and giving him a few pointers, passing down what he learned from eustace, mostly, and as fellow shitheads i think it would be real cute if they started to become friend a little bit before, y'know, (vague hand gesture)