Why are Horwitz and Wakefield (The Loss of Sadness) skeptical of the increasing of medicalization of sadness in psychiatry?

Why are Horwitz and Wakefield (The Loss of Sadness) skeptical of the increasing of medicalization of sadness in psychiatry? What evidence do they present to make their argument that sadness is "normal," and not something that always needs to be treated by a medical professional as a depressive disorder?

http://www.thedivineconspiracy.org/Z5278A.pdf (The Loss of Sadness. Chapter 2 The Anatomy of Normal Sadness).