It's rude to throw someone else's food away.
Jack just brought home a lot of groceries and the fridge is already quite full.
Jack wants to make some room in the fridge.
Jack reorganizes the fridge and throws away some of his own old food which frees up space.
Jack throws out some of his roommate's old leftovers to make room in the fridge.
Jack's roommate is happy the fridge is clearer.
Jack's roommate gets upset because he was still going to eat the food.