I HATE (= love, it makes me sob) THAT MOMENT.
Shigeru had been throwing hints EVERYWHERE that he didn’t intend to continue Pokémon League Challenges (while receiving his Hoenn dex, when he had other one-on-one talks with Satoshi by the lake, and it was regularly pointed out how Shigeru just seemed to be acting strange).
And then, Shigeru announced that he would quit battling to turn to research, because it had been something he had been interested in for a while - he was always presented as someone who knew things.
And everyone congratulates him and gives him words of encouragment.
And Satoshi is staring at him and not understanding anything.
Because, Shigeru is a childhood friend. He said it himself. And for all we know, he may have been his only long-term friend during childhood, maybe his best friend. They drifted apart, but they were still tied to the same goal, going in the same direction.
And Satoshi doesn’t understand that no, they have different paths, their interest in Pokémon doesn’t have to express itself in the same way: that, basically, he wasn’t in Shigeru’s head for a long time. This scene is just screaming that, basically, Satoshi is realizing that he doesn’t know everything about Shigeru (anymore?).
(Words of encouragment, Hazuki’s “You will always be rivals” and handshake with a smile come after; but at that moment, at the moment Shigeru announces it, Satoshi is left behind, even Kasumi and Takeshi understand, Satoshi doesn’t, and it’s HARD.)