1) HN is maybe one organism if you zoom out enough, but it consists of people with wildly different opinions, you'll have capitalists arguing with anarchists here, any post is bound to have both sides, no sides and every side, all on the same page 2) it's easier to complain about stuff, somehow. Not sure why, or if it's extra prominent on HN in particular, but people tend to start thinking "Why am I against this thing?" and then write their thoughts, rather than "Why do I like this thing?". Maybe it is more engagement to write something that can be challenged, and people like when others engage with them, so they start to implicitly learn to be that way.
I think pessimistic and cynical reactions are the literal lifeblood of news aggregator comments sections. It's been like this for as long as I can remember across as many aggregators as I've ever used.
Part of the problem is that news aggregates reward people who comment early, and the earliest comments are the kneejerk reactions where you braindump thoughts you've had brewing but don't have anywhere to put. (Probably without actually clicking through.)
Another part of it is just psychology. People seem much more inclined to join discourse to make objections than to pile on affirmative comments, which generally an upvote suffices for.
It's also partly the site's culture. Not saying it's wrong, because it adds some noise and not much new info, but I've been downvoted before for posting comments like "Thanks for saying this!"
I agree in general except HN also rewards quality a bit more. All new comments get a few minutes to bask at the top of their subthreads. So a really good, late comment can still get to the top and stay there.
To a degree, the comment ranking algorithm helps, though long/fast threads do often leave brand new replies buried upon posting.
Still, I believe what makes HN unusually nice is just the stellar moderation. It is definitely imperfect, but it creates a nice atmosphere that I think ultimately does encourage people to try to be civil, even though places like these definitely have a tendency to bring out the worst in people. Having a deft touch with moderation is very hard nowadays, especially with increasingly difficult demands put against moderators and absolutely every single possible subject matter turning into a miniature culture war (how in the hell do you turn the discussion of gas ranges vs electric ranges into a culture war?!) and the unmoderated hellscapes of the Internet wrongly painting all lightweight moderation with a black mark.
I definitely fear for the future of communities like HN, because the pressure from increasingly vile malicious actors as well as the counter-active pressure from others to moderate harder, stronger, faster will eventually break the sustainability of this sort of community. When I first joined HN, a lot of communities on the Internet felt like this. Now, I know of very few.
