I secretly gave my jewish friend non kosher food
Why did you secretly give your friend non-kosher food?
I was annoyed because he took something off my plate.
You shouldn't give a Jewish person non-kosher food. It violates their consent, which is not good to do to a friend. If they found out it could be really harmful to your friendship.
What's the big deal?
It's a big deal because your friend follows a certain diet, and by secretly giving them something that violates the rules of that diet, you're disregarding their beliefs. You should always respect someone's religious and dietary beliefs.