Why does Dazai use Chuuya instead of 'you' so often?
It's a direct correlation to Japanese language that doesn't use pronouns often. You would naturally refer to the person you are speaking to by their name, to their face directly, if you needed to refer to them. You wouldn't use 'you'.
The 'you' Chuuya uses when referring to Dazai more closely compares to saying 'asshole'.
Because English speakers don't know this, they think it's a cute thing, the way Dazai says Chuuya's name to his face, and thus it bleeds into fanfic that way. Hence many fanfic headcannonize Dazai as using Chuuya's name when speaking to him, and Chuuya as using you, bastard, asshole, etc when speaking to Dazai.
An actual, real localized translation would replace Chuuya's name with the English word 'you'.
