For me it isn't the negative content that I find depressing; it's the total lack of levity in the comments around here which causes anxiety.
You practically can't comment on anything unless you're a complete expert, lest an expert get offended by your less-than-perfect understanding of something and engage in a "gotcha" eye-rolling grouchy-faced passive aggressive attack in response, instead of just explaining politely why you're wrong. Most people are happy to be corrected by experts, and I'd rather they weren't afraid to post their ideas.
We people take ourselves too seriously, methinks. In my circles, I see a lot of impotent nerd-rage boiling underneath the surface (of which I too have been guilty) and it comes out whenever someone is even just a centimeter off when trying to talk about something complicated.
edit: This popular comic describes it well: https://xkcd.com/386/ There's a bit too much of this.