I don't like sports that much and I also think sports obsessives are dull, but sports are really happening in the real world with real people. You can talk about a man who really has scored a goal or a team that really has lost a match. I don't think it's a good comparison.
I love analysing books and thinking about fictional storylines and I used to chat online about what would happen in the next books when the series wasn't finished.
But there's so much weird extrapolation and theorisation about HP and the characters online now that's barely rooted in the text and people talking about it like these are real people. I get the sense some people use fictional universes to disassociate from reality. Not to mention the inappropriate sexualisation of child characters - that's a whole other side of it.
But obviously for the most part it's essentially harmless, more so than football hooliganism.