Dogs eat poop for lots of reasons. Sometimes there’s an underlying medical problem like an enzyme deficiency or pancreatic insufficiency. Intestinal malabsorption and GI parasites are also common medical reasons underlying coprophagia.
At my practice we recommend clients bring their dogs in every six months for a stool check for parasites. Healthy dogs can wind up with intestinal parasites from eating poop, so twice-yearly stool analysis can be a very helpful tool.
A dog’s pancreas secretes digestive enzymes to aid in food digestion, but many dogs don’t make enough of these enzymes and wind up deficient. Since the feces of other animals are a good source of digestive enzymes, dogs with a deficiency will sometimes ingest enzyme-rich poop. In fact, rabbit poop is a very rich source of not only enzymes, but also B vitamins, which is why many dogs, given the opportunity, will happily scarf up rabbit droppings.
In my experience, dogs on processed dry food diets will often seek out other sources of digestive enzymes to make up for a chronic enzyme deficiency brought on by a biologically inappropriate diet.