Why do dogs eat there stool?

Dogs naturally eat their poop and other dogs poop for nutrition, so a domestic dog shouldn't need to, but the instinct may still be there or your dog may be eating too fast, making them think they are not full and need more food, if you spread out a dogs food through the day or feed it in servings so it is forced to slow its eating down it may stop eating poop, if it is a learned behavior you will have to train the dog not to do it, but that's why they do.