I'm mad that my friend got me food I didn't ask for
Why are you upset about him getting food for you?
I don't want to eat anything from other people.
You should be thankful someone thought enough about you to make a special accommodation. It is rude to reject some ones effort even when you don't like it.
What if I simply don't give a shit about anyones opinion about it.
You don't have to care about the opinion, but you can still express your thanks about it. People feel nice when they do things and are thanked for it.