Your topic question and body text aren't asking the same question but I'll try to work with it. I typically avoid commenting here but it's an interesting question.

What do you think is behind redditors' obsession with 'rational' and 'logical' discussion, and the dismissal of emotions even when they are appropriate?

"Dismissing" emotion, if it were truly possible to do such a thing to its fullest extent without a frontal lobe malady of some variety, is the easy way out. That said, it's about what it means to be passionate about a given subject. Unless you're doing something to an aberrant level (or some other "excess" taboo like illegality), most people won't have an issue with someone that's impassioned about something. Online? That's seen as a bad thing.
"You openly say that you like [x]? Well, you must be a loser/psycho/creep/etc." Meanwhile, [x] has a subreddit community that numbers in the hundreds of thousands or more.

It's the driving concept behind why I always say "Reddit users hate admitting that they use Reddit." I dunno. People tend to take a given comment on an online forum as being some kind of snapshot of the user's entire existence. Sure, they might come off as being passionate about it but even if it's less than 0.001% of their day, that doesn't matter online. Clearly they're way too invested in it! Or as I've seen spouted in arguments on Reddit: "Stop being so mad" when the person they're respond is clearly nothing of the sort. It's lazy to think that way but many people do.

And ultimately, it has little to do with being "rational" or "logical". Critical thinking is difficult. Critical thinking about emotionally-invested issues is extremely difficult. And I suspect most people have almost no idea how to even approach it.