The processed canned food they eat gets stuck to their teeth and causes decay. Giving your cat dry food can help to clean it's teeth. In the wild cats eat raw food which does not stick like processed food does. They might also chew on hard things in the wild to clean their teeth. Give your cat more dry food than canned. At this point however, your cat should see a vet for it's dental issues. If it's teeth hurt, it will not eat the dry food.