The narrator and the text do not explicitly explain the reason for Bartleby’s behavior, leaving it open to interpretation.
As an example of clinical depression
Bartleby shows classic symptoms of depression, especially his lack of motivation. He is a passive person, and good at the work he agrees to do. He refuses to divulge any personal information to the narrator. Bartleby's death is consistent with depression—having no motivation to survive, he refrains from eating until he dies.
As a reflection of the narrator
Bartleby has been interpreted as a "psychological double" for the narrator that criticizes the "sterility, impersonality, and mechanical adjustments of the world which the lawyer inhabits." Until the end of the story, Bartleby’s background is unknown and may have sprung from the narrator's mind. The narrator screens off Bartleby in a corner, which has been interpreted as symbolising "the lawyer's compartmentalization of the unconscious forces which Bartleby represents."
Psychoanalyst Christopher Bollas says the main focus of the story is the narrator, whose "willingness to tolerate [Bartleby's] work stoppage is what needs to be explained ... As the story proceeds, it becomes increasingly clear that the lawyer identifies with his clerk. To be sure, it is an ambivalent identification, but that only makes it all the more powerful."
Bartleby's employer provides a first-person narrative of his experiences working with Bartleby. He portrays himself as a kind and generous man. When Bartleby's work ethic declines, the narrator allows his employment to continue. He portrays himself as tolerant towards the other employees, Turkey and Nippers, who are confrontational in the afternoon and morning, respectively. The narrator is torn between his feelings of responsibility for Bartleby and his desire to be rid of the threat that Bartleby poses to the office and to his reputation on Wall Street.
"Bartleby, the Scrivener" alludes to Jonathan Edwards's "Inquiry into the Freedom of the Will" and Jay Leyda, in his introduction to The Complete Stories of Herman Melville, comments on the similarities between Bartleby and The Doctrine of Philosophical Necessity by Joseph Priestley. Both Edwards and Priestley wrote about free will and determinism. Edwards states that free will requires the will to be isolated from the moment of decision, in which case Bartleby's isolation from the world would allow him to be completely free. He has the ability to do whatever he pleases. The reference to Priestley and Edwards in connection with determinism may suggest that Bartleby's exceptional exercise of his personal will, even though it leads to his death, spares him from an externally determined fate.
"Bartleby" is also seen as an inquiry into ethics. Critic John Matteson sees the story (and other Melville works) as explorations of the changing meaning of 19th-century "prudence." The story's narrator "struggles to decide whether his ethics will be governed by worldly prudence or Christian agape." He wants to be humane, as shown by his accommodations of the four staff and especially of Bartleby, but this conflicts with the newer, pragmatic and economically based notion of prudence supported by changing legal theory. The 1850 case Brown v. Kendall, three years before the story's publication, was important in establishing the "reasonable man" standard in the United States, and emphasized the positive action required to avoid negligence. Bartleby's passivity has no place in a legal and economic system that increasingly sides with the "reasonable" and economically active individual. His fate, an innocent decline into unemployment, prison, and starvation, dramatizes the effect of the new prudence on the economically inactive members of society.
Failure to communicate
An element of the story that leads to tragedy is the failure of Bartleby and his employer to communicate with each other. One day, Bartleby simply stops following orders. From this point on, his reply to any order or request is passive resistance. But the rebellious employee seems either unable or unwilling to explain what motivates his sudden rebellion. On the other hand, his employer is evidently unable to comprehend that Bartleby may have reasons to resist his orders. The employer's refusal to accommodate Bartleby or his needs is what leads to Bartleby's tragic end. 
Melville devotes time to introducing the office environment which Bartleby joins, and the nature of his employer and his co-workers. The employer is an elderly lawyer and describes himself as unambitious. He previously had tenure as a "master" in the New York Court of Chancery. He employs scriveners (law-copyists) to deal with his legal documents. 
The eldest scrivener at the office is nicknamed Turkey. He is in his late 50s, and the narrator implies that he is a heavy drinker. He spends his lunch hour drinking. The younger scrivener is nicknamed Nippers. His employer considers him overly ambitious. The only other employee besides the scriveners is an office boy. He has been nicknamed Ginger Nut, because he brings ginger nuts for the scriveners. 
Bartleby is initially hired because he appears "sedate and respectable" in demeanor, unlike the other two scriveners. For two days, Bartleby is an industrious worker. He works quietly behind a folding screen that prevents him from maintaining eye contact with his employer. His only view through the office window is a wall. 
On his third day at the office, Bartleby is asked to proofread legal documents. He refuses a direct order from his employer for the first time. His employer considers firing Bartleby, but changes his mind when he notices Bartleby's "perfect composure". Days later, Bartleby refuses a similar order. His employer demands an explanation, but Bartleby offers none. The employer points out that his initial order was reasonable, and tries to appeal to Bartleby's common sense. When Bartleby again fails to obey, the employer has his three other employees work on persuading Bartleby. Bartleby's behavior does not change. 
As days pass, Turkey offers to beat up Bartleby for his employer. The employer refuses to resort to violence. By chance, the employer finds out that Bartleby has moved into the legal office. Bartleby has no other home of his own. The employer pities Bartleby for his loneliness, but he also feels fear and revulsion for Bartleby. 
Upon closer observation of Bartleby's behavior, the employer notices that it is stranger than he previously thought. Bartleby no longer reads anything, and makes no effort to converse with other people. He spends much of his time staring at walls, and Bartleby's blank gaze implies that something is off. 
The employer's efforts to get rid of Bartleby
After Bartleby refuses to explain anything about his personal life to his employer, the employer becomes determined to get rid of him. Up to this point in the story, Bartleby has kept copying legal documents. He simply refuses any orders about proofreading. Afterwards, Bartleby stops his copying work. The employer waits for a few days to see if Bartleby is willing to resume work. When there is no such sign, he gives a deadline to Bartleby. The scrivener must vacate the premise within six days. 
With the end of the deadline, the employer fires Bartleby. He pays Bartleby his full wages, plus twenty dollars. The next day, Bartleby is still in the office. The employer decides against using physical force or calling the police. He tries to ignore Bartleby's unwanted presence in his office, but soon realizes that people are gossiping about Bartleby's behavior. He fears that his professional reputation is at risk, but again decides to not confront Bartleby. He instead moves into a new office.