Cats don't change their behavior just because, there is usually a good reason. Think about how things have been recently. Have you changed work hours or begun seeing someone new? Have you been petting a stray outside? Have you recently had new neighbors? Many things can change the behavior of your cat because they like things to be their way and ordered. Once you can identify the cause, you can act accordingly to fix it. One way is to wash your hands after petting another animal. Another is to make sure your cat has plenty to occupy him while you are busy or out of the house. Be sure he can see out a window, so he doesn't feel trapped and be sure he likes his food and litter box.