Especially in the case of Show HN posts, like you mention, it seems like people put themselves out there and most of the comments shoot them down, often without really exploring or trying to understand what they've spent their time on. Negative feedback is often quite helpful, but knee-jerk negative feedback almost never is.
Obviously it's a tough game, and you've got to have thick skin, but it did used to seem like this was a place to go to learn to succeed, and lately it kind of seems like a place to go to learn that everything sucks.
As for what to do about it, I'm not sure. I'll just keep reading it every day until a new, smaller, more constructive community pops up. I am (and I suspect many others on this site are as well), basically just an education junkie, and as the community becomes less educational, it becomes less attractive than alternatives.