You might find this greedy, but I'm not going to share food with my best friend
People usually share food with their friends. Why won't you share food with your best friend?
I want my food all to myself.
You should share things with your friend. It's only right to do that. Not doing so may make you come off as greedy.
Can't they buy their own food?
It doesn't matter if they can buy some food. It's just nice to share with friends. I'm sure you would want them to share with you wouldn't you?