Scientists have no idea why cats purr but they do have theories. One humorous theory is that cats purr because they know they can attack you at any moment. Another theory is that cats purr because it’s similar to humans humming.
Yet scientists believe that cats may purr for a variety of reasons from contentment to fear and anxiety. Whatever the reason, your cat will likely purr just to keep you confused as to why it’s doing it in the first place.