In The Catcher in the Rye, JD Salinger warns that depression is dangerous and can drive people into insanity.
This is arguably exemplified in every word of prose in the entire novel, but it most obviously appears in Chapter 20 after Holden drops a record he bought for his sister, the gift breaking into several pieces. By Chapter 20, Holden is at his most emotionally vulnerable, having lost all of his money and friends with nobody at fault besides himself. In a moment of desperation, Holden lays down on a nearby bench. He goes on, writing “I thought I’d get pneumonia and die. I started picturing millions of jerks at my funeral and all” (200).
Because Holden is in such a state of depression that he starts to contemplate his own suicidal demise, this explains how depression can lead to insanity.