Many folks have talked about optimizing for engagement and while I do think this is a factor, I don't think this is the cause. HN has a pretty simple set of algorithms and yet it's largely followed the same trend of negativity and despair.

I'm partial to what I call the "doomer effect". If you feel sad or disenfranchised you're just much more likely to spend time online talking. A person with a fulfilling social life and a job they feel satisfied with is going to spend time socializing with friends and working on their job. They may read online feeds, they may write very occasionally, but that will be the extent of most interactions. On the other hand the folks that feel the most negative tend to spend inordinate amounts of time online posting about how sad and angry they are.

In a real life conversation when a depressed friend gets on their soapbox most folks give the friend a subtle clue that it's time to move on. If that clue isn't heeded the friend often gets targeted 1:1 offers for help and can eventually just be shut out of the group altogether for being too negative. Nothing like this happens offline. If you're in a bad place, as long as a forum will have your posts you can post to your heart's content. And oh boy is it finally the time to let your hair down and tell everyone how f-ed everything is on the inside. Constantly.

This makes a flywheel. People who want to avoid negativity start avoiding negative online spaces. People who are fine with it or feel similarly keep reading. Eventually negativity becomes the default.

"Misery loves company" is much older than the Internet. There's no doubt to me that the net makes it that much worse.