PTSD among combat vets, for example, can have a person chronically frightful while they live in a safe environment actively preventing them from properly apprehending their current environment. Most people who want to rid themselves of their mental illnesses aren't experiencing it as a signal that their outlook is wrong, their mental illness is actively hampering their ability to properly engage and function in their life.

And for illnesses with a stressor: sometimes a person is not empowered to resolve it.

You shouldn't have too much faith in your own biology, it is far from perfect.