Although Bartleby never explains why he has refused to do work, Melville gives us many clues. The best of these clues comes on the last page, when we learn that Bartleby had worked in the dead letters office. This office is symbolic of the lack of connection and communication between people who are caught up in a society devoted to work. The narrator and his employees are not friends; they do not share life together. This is emphasized by the fact that the narrator only refers to them by nicknames, as if he doesn't know or doesn't care about their real names. They are employees only - not friends. The narrator himself isn't known by a name - he is only the lawyer. Again, he is known by his job, not who he is as an individual.
The depressing nature of the dead letters office seems to have seeped into Bartleby by the time he comes to work for the narrator. He has no life outside work - he doesn't even seem to have a home outside work. However, he must have realized how pointless the work is; just like the time put into those dead letters had become pointless. And so, he starts to refuse to do things. Bartleby has found work to be insignificant. And because he has nothing but work, he will soon find life to be insignificant. By refusing to do the work, he is making a stand against a society that has placed so much importance on it. Unfortunately, he is also dooming himself.
Bartleby doesn't refuse to do work: He prefers not to. The problem is that instead of firing Bartleby, the narrator reacts in astonishment the first time he gets that response. Whenever he is asked why he won't do the work, the only answer Bartleby gives is, "I prefer not to." The only answer we can come up with for the refusal, then, is that he just doesn't want to do it.
Bartleby is like a man I used to work with. When our office upgraded from paper and pencil to desktop computers, he rebelled in a Bartleby-like way. Rather than move his monitor away from the window or ask for an antiglare screen, he opened an umbrella over his desk to cut the glare. When told he couldn't do that, he started wearing sunglasses in the office. My former coworker was used to editing a hard copy manuscript with a colored pencil. He had a hard time adjusting to the new technology. Likewise, Bartleby is fine as long as all he has to do is copy what his boss has written. When asked to do something different, he just can't--or doesn't want to--adjust.