When I was teaching, my students often had trouble with "Bartleby" because they could not understand why Bartleby acted the way he did; they also were not sure why the narrator didn't throw him out immediately. So, I always tried to tackle these two problems of motivation at the beginning.
The story is difficult because it marks a transition between fabulistic stories, in which characters are two-dimensional representations, and realistic stories, in which they are presented "as-if" they were real. As a result, Bartleby seems to be a fabulistic character, while the narrator seems realistic.
There is no way Bartleby can answer the question, "what is the matter with you?" because Bartleby has no "matter"; that is, he can only react as a two-dimensional representation of passive rebellion. In Hamlet the Queen says to Polonius, "More matter, less art." But Polonius is no more "matter" than Hamlet, who is a "poor player" who cannot "act" because the only thing he can do is "act."
The one place in Melville's story when Bartleby comes closest to answering the question about what motivates him is when he has decided to do no more copying at all and the narrator asks him why. Bartleby, standing looking out the window at the blank wall, says, "Can you not see the reason for yourself?"
The narrator, an "as-if-real" character thinks there is something wrong with Bartleby's eyes. Bartleby, a two-dimensional figure, is referring to the metaphoric representation of his problem--the blank wall. However, it makes no sense to tell an "as-if-real" person that the reason one has decided to do nothing is because of a wall.
To do so is to be accused of madness (as Bartleby indeed has been accused of), for it means to mistake a mere object in the world (the wall) for what one has taken the object to mean (meaninglessness, nothingness, blankness, loneliness, isolation). As Polonius tells the Queen: "Your noble son is mad: Mad call I it, for to define true madness, What is't but to be nothing else but mad? But let that go."
Although the narrator cannot identify with Bartleby's metaphoric mistake, he feels the power of Bartleby's loneliness and need. He knows that the only cure for Bartleby's isolation is brotherly love, but he is unable to grant that love on Bartleby's terms--that is, that he completely lose himself, give up everything. For the metaphoric character, it is all or nothing at all; the "as-if-real" character, however, feels he must exist in the practical world.
Melville's story is ambiguous and mysterious because, like Hawthorne's "Young Goodman Brown," it is both fabulistic and realistic at once. The wall is a "dead letter" for Bartleby because it signifies "nothing," and "nothing" is that which he cannot bear. Bartleby is a "dead letter" for the narrator, because, although he has intuitions about who or what Bartleby is, he cannot "go all the way" into that realm of madness, the metaphoric, and the sacred that Bartleby inhabits; he can only tell the story over and over, each time trying to understand
Tomorrow: Hawthorne's "Young Goodman Brown".