I've ventured back lately. What I've noticed is that commenting on Reddit makes me angry and jumpy for hours afterward. It's a deeply confrontational place. The first time I visited Reddit I was shocked by what a nasty place it was. Then I adapted to it. I got good at "Redditing." Which means it turned me into a nasty mutherfucker. Thing is, so's everybody else so you get so you don't notice it any more. I enjoy not being so nasty. It's taking me time and I still relapse, but I'm all about any place where conversations can be rewarding without being confrontational.
I think it has more to do with Reddit's efficiency of pushing "cream" to the top when "cream" means "anything people feel passionately about." When Reddit was smaller, that "cream" tended to represent the values of bookish, erudite computer programmers. It now represents a broad swath of humanity, which means the things that are the most appealing are the things that are the broadest - base emotions. Reddit pushes "happy" very hard. However, it also pushes "outrage." My first 3 digit comment was a pure vitriolic spew of hatred - it got 600 upvotes, back when 600 upvotes was unheard of. Most of the rest of them weren't - but Reddit has definitely been appealing to a broader, baser audience for quite a while now.