Why have we built an internet culture that encourages negativity? If you are on a message board, replying that you agree with the original poster is often taken as pointless, content-free post-count building. Blogging “look at this great post” is sometimes described as being unable to develop worthwhile content of your own, the sort of thing you can only occasionally indulge in. If someone replies with “Great post!” there is no point to anyone doing so again, right? The only thing you can post agreement with is that someone else is wrong. You can agree with each other that someone else is an idiot.
You are only to post if you have additional content to add. Generally, that means you disagree. Even if you agree, you are extending the post away from where the original poster took it, which can easily come across as a slight in the sea of negativity.
The major exception I can think of is posting examples and stories. “Yeah, something like that happened to me one time…”
Wayback machine: Shiny Happy Week