Seiroku not coming back from the war was so traumatic for Yosuke that it completely rewired his brain - and changed his hair colour as per 2nd episode’s opening sequence - so that his already strong feelings (of love? adoration?) for Seiroku turned into an obsession with the promise between them. The promise he clings to as the only thing that still connects him to Seiroku and which he considers his and his alone and doesn’t want to share with anyone. I mean, liking a handsome guy with a handsome voice is basic, but at least Yosuke’s not normal about it.

Anyway, sorry for rambling, he’s just so entertaining to watch, and I’m happy to see people acknowledging it instead of finding him off-putting enough to consider dropping the show.