Source: https://news.ycombinator.com/item?id=45239085

One user hilyan commented that we are afflicted with RNT because:

... human brains are naturally predisposed to negative thought than positive thought because our brains are essentially evolved prediction engines. And because it is often easier and faster to lose something than gain it (e.g. it is usually less urgent to act on the signs of deer you might want to hunt and eat, than the sign of a tiger who might want to hunt and eat you), our prediction engines have a bias toward negative prediction.

to which another user theptip said:

Sounds like “loss aversion”

This makes so much sense to me. We over-index potential losses as a survival heuristic—so our “default prior” tilts negative.