What are the reasons that prompted Oliver to ask for more gruel?

The primary reason that Oliver asks for more gruel (which is similar to oatmeal) is because he is hungry. The board members of the workhouse where Oliver lives meet and decide that "the poor people like" the room and board they receive. Supposedly, the poor people enjoy being in a place where their meals are provided for and they can live without expense (besides their labor) (ch. 2). The board decides to feed the poor people less food to lighten the expenses of the facility and to encourage the poor people to get out of the workhouse. Dickens describes the miserable effects of the board's decision:

"It was rather expensive at first, in consequence of the increase in the undertaker's bill, and the necessity of taking in the clothes of all the paupers, which fluttered loosely on their wasted, shrunken forms, after a week or two's gruel. But the number of workhouse inmates got thin as well as the paupers . . ." (ch. 2)

The board's decision to reduce the food portions has grave effects: many die, and those who live have to get smaller clothing to fit their emaciated bodies. This starvation policy leads the boys, who "generally [have] excellent appetites" to "suffer . . . the tortures of slow starvation for three months" (ch. 2). In fact, one of the boys claims he is so hungry that

"unless he had another basin of gruel per diem [day], he was afraid he might some night happen to eat the boy who slept next him" (ch. 2).

Though the boys aren't truly considering cannibalism, they are very, very hungry. This is Oliver's primary motivation for asking for more food.
Their great hunger leads the boys to hold a "council" where they elect (through lots, or by rolling a dice) Oliver to ask the master for more food. Though he realizes that he will likely get in trouble for his discontent with the portion of food offered to him, he asks because he is sincerely hungry. He also asks for more gruel to (hopefully) benefit his friends with a bit more food. His wish is not granted, and he does get in trouble for making this request.
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