I knew this in the back of my mind for some time now, but I didn't really look into it until I saw this feature on the Kitchn this week. Rennet is an animal-derived enzyme which is used in the acidification process of cheese-making. Unfortunately, you can't get rennet without killing the animal. When you shop for cheese, try to look for "vegetal rennet" or "plant-derived rennet" in the ingredients list. The Kitchn's guide gives some further buying tips.