Yousuke is a little jealous of the fact Sagara is getting along better with Katsaki-san, than he's doing with Okita. Katsaki-san was in trouble when her top slipped but she was protected by Sagara again, and he didn't try to peek at her embarrassment like most other boys would have, it made her fall for him even more, he let Okita help her without making it obvious or trying to do something to embarrass her.