Why do so many people think Dazai is a father figure to Atsushi?

I don't really get it, honestly. They're only four years apart, and Dazai doesn't really act mature enough around Atsushi to be a father figure in his life. Originally, Atsushi was even going to be the same or a similar age as Dazai before Asagiri changed it.

Plus, making Dazai a father figure to Atsushi kind of undermines his relationship with the Headmaster, his canonical father figure. Same with Kunikida too.

I kind of understand it to some extent because Dazai is Atsushi's mentor, but that doesn't really make him fatherly towards Atsushi either way.