A negativity bias or cognitive distortion is particularly about thinking things are worse than they really are. If things are really bad all of the time, then it's not a distortion or reality. It would be strange to expect someone who gets punched in the face everyday to develop a positive attitude about getting punched in the face.
But if that same person sees an act of kindness - such as someone giving a child some candy - and their first thoughts are that the child is going to get diabetes or be poisoned, then that is a distortion. Your mind is trying to find the negative angles of a positive action.
The reason it is problematic is that the human species is a social animal. If we see negativity everywhere we lose the ability to trust. If we lose the ability to trust, we grow more solitary and disconnected. That is a weakness in social animals. We're not built to function in solitude. Too much positivity also leads to issues as well though. So forcing positivity everywhere isn't really a good fix either. You need to be able to step back from situations and view them from both angles.
