It's rude to hog all of the lefteovers.
Patrick is browsing through the fridge when he notices the pizza he and his roommate ordered the night before.
Patrick wants to get himself something to eat.
Patrick takes a couple slices of pizza and leaves the rest for his roommate.
Patrick takes all the rest of the pizza and puts it in the oven to reheat it for his lunch.
Patrick's roommate thanks Patrick for saving him some pizza and is also able to enjoy some for lunch.
Patrick's roommate doesn't find any pizza when he's looking for something to eat and thinks that Patrick is being really greedy.