You're talking about coprophagia. Theories abound, but no one seems to know why some dogs do it. I've had lots of dogs, but my latest one was the first to exhibit this behavior as a puppy. Our vet recommended putting a dash of MSG in her food. She eventually stopped. Whether it was the MSG, or whether she just grew out of it, I don't know. Here is a link that might help:
I have Chihuahuas and Poms, they did it. I stopped them by adding a little Pineapple Juice in their water. They no longer eat their Poop. They started eating the Cats, so I put some juice in his water as well. They stopped immediately.
my friend has a german shepherd mix that did it... real bad! And it was because she was being fed very low grade dog food.... and the vet said that basically what was going out was VERY similar to that that was going in... so that is why she would eat the poop. She wasnt getting enough nutrients from the food... so she would eat the poo..... totallly yucky, but that is what the vet said. They upped her to a premium dog food and that stopped it completely. Good luck! He he he.... one time we brought her to petsmart for a bath and grooming... and well they brush the teeth too... poor lady! She had poo teeth.... He he he.... TOO FUNNY! Yucky but FUNNY!