We never find out definitively why Bartleby shuts down and decides he "prefers not to" do any work whatsoever. However, the sympathetic and kind-hearted lawyer employer who takes an interest in him comes up with a theory that he presents at the very end of the story.
The lawyer has discovered a "rumor" that Bartleby once worked at the Dead Letter office in Washington, DC. This office is a central clearing house for all the letters that never reached their destined audience. The lawyer surmises that having to deal with the futility of communication—all the fervent and heartfelt letters that never arrived at their destinations—had a depressing effect on Bartleby's already despairing mind. This job made him find a sense of futility in all human endeavor. As the lawyer puts it:
Conceive a man by nature and misfortune prone to a pallid hopelessness, can any business seem more fitted to heighten it than that of continually handling these dead letters, and assorting them for the flames?
(The entire section contains 3 answers and 561 words.)