Mental illness is a literalized metaphor.
Misery and tragedy have always existed. It is only in the last few hundred years that misery trauma, and tragedy have been medicalized into what we refer to as mental illness.
Anything can be medicalized. Money hungry psychiatrists will increasingly medicalize more and more human experience until a significant number of humans see the damage that this dehumanization creates.
Read books by psychiatrist Thomas Szasz if you want to learn more. Read his book The Medicalization of Everyday Life.