I guess the question is why your priors are so far weighted to the side of negative outcomes. If we're talking about yet undiscovered effects of something it seems equally plausible for those effects to be positive. Aspirin is a pretty good example of this where we keep discovering more positive effects. And I can understand somewhat the bias toward the state of nature but there's lots of examples where our deviations were positive, the biggest one being the cognitive effects of cooking food.