I'm going to take food off my friend's plate without asking first.
Don't you think taking food off their plate is rude?
He has plenty of food. Why should he be the only one to get to eat it?
You should ask before taking something from someone, even a friend. If you are nice about it and ask if you can have some of their food, they will probably give some to you. If you just take it without asking, they will think you're selfish and like you less.
I don't care. I want his food and I'm going to do it anyway.
I would find it very strange if a friend just kept eating off my plate without asking first.  I think he probably won't remain your friend for much longer if you do it.