Why does Septimus Warren Smith die in Mrs. Dalloway?

In Virginia Woolf's novel Mrs. Dalloway, the character Septimus Warren Smith dies by suicide after being told he must be committed to a psychiatric hospital.1

As a World War I veteran suffering from shell shock—now recognized as post-traumatic stress disorder—Septimus experiences profound internal suffering that those around him fail to understand. When faced with the prospect of institutional confinement, he chooses to throw himself from a window rather than allow his soul to be imprisoned.1

Ultimately, Septimus represents the tragic breakdown of a society unable to address the psychological trauma of war. His death serves as a powerful contrast to the protagonist, Clarissa Dalloway; while she conforms to societal expectations to survive, she later identifies with Septimus’s choice, viewing his suicide as an act of courage that preserved his integrity against the confinement she feels in her own life.1