It was really a painful scene and reminded me a lot of my childhood, specifically an incident when I was living with the Odhners, a family of eight. I snuck a piece of cake (made from a mix I'd bought) because it seemed silly to hide it and I wanted some, and then was told because I'd had some, Stephen wasn't going to be able to have any. It was just such a sign of poverty, that to have any would be denying someone else, that the simple act of eating was taking away from someone else. I'd never had a cake that had more than two people to eat it, so I didn't see it as a situation of scarcity, and yet, with a change of environment, suddenly I was a thief.
This situation of scarcity and subsequent hoarding problems is a hallmark of growing up in the environment I did. You don't want to take something you're not supposed to have. Similarly, anyone giving you something is suspect. Will you owe them? Will they hold it against you? In retrospect, getting to a point where I could give without expecting to receive, and, more importantly, receive without having fear of what I would be expected to pay in return, has been one of the great pleasures for me of getting out of that environment. I can still see it in my family, that fear that any favor accepted, no matter how desperately needed, will put you into a situation of indebtedness that you may not be willing to experience. Being really poor is saying no to something you need badly because you can't trust that it would ever be offered to you without strings attached.
Rambling, I know. I may try to clear it up later but it's 1 AM and I must get some sleep. I know I misquoted all of the dialogue (including the name of the son and the cost of the cake), but hopefully it's enough to convey the idea - the script is unfortunately not available for free online.