A lot (most) social media is optimized for engagement which is really just another way if saying it rewards things that get attention with more attention. On mainstream platforms that is largely driven by content curation algorithms, but even simple up and down votes have the same effect.

It turns out humans are more sensitive to bad things than good things. We care more about risk or punishment than we do about reward, more about injustice than justice, etc. So we pay more attention to bad outcomes, which means social media amplifies bad things more than good things.

As far as personal posters go, I think there are people who desire engagement and have basically trained themselves to be as negative as possible because that negativity is rewarded by more engagement.