From Susan Sontag’s Illness as Metaphor

Theories that diseases are caused by mental states and can be cured by will power are always an index of how much is not understood about the physical terrain of a disease.

Moreover, there is a peculiarly modern predilection for psychological explanations of a disease, as of everything else. Psychologizing seems to provide control over the experiences and events (like grave illnesses) over which people have in fact little or no control. Psychological understanding undermines the ‘reality’ of a disease. That reality has to be explained.

Susan Sontag, Illness as Metaphor (New York: Farrar, Straus and Giroux, 1977), p. 55.