I'm not sure that the case, the brain is very complex so there lots of places thing can go wrong.For example damage in the limbic system could cause increased fear responce or damage in part of the brain that could down regulate the limbic system. We tend to acknowledge this in the case of brain injury. But there does seem to be a bias when it comes to neurological defects that don't have any known direct causes. But it super possible to just have a brain that emotion processing is abnormally predisposed to fear responses to that point that is should be considered a mental illness