The answer to this question can be found in Chapter 18, when Oliver has been reclaimed by Nancy and is living once again with Fagin. The Artful Dodger and Charley try to convince Oliver to embrace the life of a thief, saying that it will gain him a life of ease and limitless riches. They portray it in a very romantic way, such as the Dodger does in this quote:
"Look here!" said the Dodger, drawing forth a handful of shillings and half-pence. "Here's a jolly life! What's the odds where it comes from? Here, catch hold; there's plenty more where they were took from. You won't, won't you? Oh, you precious flat!"
To Dodger and Charley, the "jolly life" of thieving and working for Fagin means that they have independence and are not dependent on others, and that they have easy access to money. Dickens presents these two characters as being so steeped in their surroundings that they are unable to morally question or challenge what is going on. Oliver, by contrast, is a character who is so uniquely good that he is unable to accept such an unscrupulous path for himself in life, and this is something the other two boys find completely unbelievable.