Bartleby suffers from clinical depression. The author notes as much in their conclusion, where they explain that Bartleby's soul was crushed slowly and steadily by lonesome work in the dead-letter room of the post office.

Bartleby never refuses to do work in such a plain way. He explains himself. "I would prefer not to," he says, and indeed he means it. He's not sure what he would prefer to do other than sigh in malaise and look out the window, but routine paperwork is certainly not it. Remember that Bartleby's job would today be done by a Xerox!

I too have been thinking of Bartleby through this discussion :D