a theme of negativity and judgement permeating across all subreddits

This is my two cents but most people (I'll say anyone 20+ older even up to 60 year olds) are insecure about themselves. Most adults who I've met have some sort of neurosis whether it's anxiety or depression or low self esteem.

You can take your pick because they can all have an effect on your personality and sense of well being. And most people who spend enough time creating a community or are in one on reddit are probably escaping some other reality they don't want to confront on some level. And the reason why so many people have such a low value of themselves is because they spend alot of time trying to prove strangers on the internet wrong. And they feel the need to tear down others to try and get a sense of accomplishment or satisfaction. People will often go to one extreme to try and counter an opposite as a way of compensating. There is a gladiatorial sense of competition when it comes to being "right" and it pushes people to do things they wouldn't normally do in real life. It's why you have people doing witch hunts or sending private messages to you with scathing remarks you would never otherwise say.

The notion of up voting feeds into this aswell and it creates a pavlovian response in our minds and when we use something alot it will give us either positive or negative associations. If I get downvoted into oblivion I'll be more willing to delete my post or edit it and feel like shit compared to being upvoted thousands of times and being on the front page. When we don't get that immediate response our minds tend to assume the worst or we feel starved of attention.

But what you're talking about is a microcosm of a much bigger problem most western countries aren't prepared for: being able to provide quality mental health care for their populations. I really do think it poses a bigger problem than other hot button issues like terrorism or financial disaster because we're all at risk. And I think that technology has either highlighted how prevalent mental illnesses are or they have on some level played a role in developing the early stages of it due to people depending on it as a crutch for life.