Roommate Dispute, Please Help.?

[Ryan]

So about two months into school starting, I went to the market on campus to buy four boxes of cereal with my meal-plan points. About two days later, I woke up and found one of my cereal boxes about 80% eaten next to my roommate's desk, and I know for a fact that he ate it with his hands, so I obviously didn't want the rest of the cereal. At the time, I told him to buy me a new box of cereal and when he didn't, we proceeded to hide the cereal around the room for a week as a joke, but I eventually dropped the whole thing. Now it is about two months later, and a week before we go home for Thanksgiving Break. I need one box of cereal to get me through the week for my breakfast, so as my roommate was walking to the market I told him to get me a box of cereal and reminded him of what happened. I no longer have points and neither does he, so we would have to spend cash to get the cereal. We then proceeded to get into a screaming argument about how he doesn't owe me cereal because I dropped the argument and because we should share things as roommates. I'm trying to become used to the idea of sharing with others but it's hard when you're roommate is from California and never brings anything back from home because he cant, so he mooches off of what I bring. My definition of sharing also includes the fact that you actually shared the food, and it wasn't one person taking from someone else so that they could gain no satisfaction from what they paid for. I know that this appears to be a stupid and pointless argument, but I'm trying to say that it is bigger than that. Please offer your opinions on whether or not my roommate should buy me a box of cereal. I mean, isn't it common courtesy to just replace the thing that you took from someone else?