A dog eats it's own poop when it has digestive problems.
When a dog starts eating it's own poop it's doing so to correct digestive problems and although disgusting it's nothing to be alarmed about and is a very healthy thing for the dog to do.
It also doesn't mean anything serious with a dogs digestive system when a dog starts eating their own poop so you don't always need to take the dog to the vet unless absolutely necessary.
The dog eating the poop will usually solve the dogs digestive problems.