> It's human nature: https://en.wikipedia.org/wiki/Negativity_bias. Everyone does it, but we perceive other people as doing it more than we do, which is itself a variation of the bias. You can even see it in the title of the OP, in the word "overwhelmingly". That's excessive: the negative bias is noticeable, but if you look closely, it's not overwhelming. (To make up some numbers, it's more like 60-40, not 90-10.)
However, it often feels as if it is overwhelming; in fact, one or two datapoints, plus negativity bias, are enough to create just such a feeling. The feeling gets expressed in ways that trigger similar feelings in other people, so we end up with a positive* feedback loop.
The interesting question is, what factors mitigate this? how do we dampen negativity bias? or, how do we get negative feedback into our positive feedback loop of negative affect? That must also be happening all the time, or we'd be in a "war of all against all", which isn't the case, though (again) it may feel like it.
* ['positive' in the sense of increasing; a positive loop of negative affect!]