Quick answer: Coprophagia is common in dogs and usually caused by instinct, attention-seeking, or learned habit — it's rarely a sign of serious illness.
Mother dogs eat their puppies' feces instinctively to keep the den clean. Puppies explore by tasting everything, including feces, and most grow out of it by 9-12 months. If you have a young puppy, this may simply be a developmental phase that resolves with time and cleanup.
Under-stimulated dogs, dogs seeking attention (even your disgusted reaction counts as attention), or dogs with dietary deficiencies are more likely to engage in coprophagia. Dogs also frequently eat cat feces from the litter box because cat food is protein-rich and highly palatable to dogs.
Immediate supervision and prompt cleanup is the most effective approach — no access means no behavior. Commercial deterrents (For-Bid) can make feces less appealing. Increasing feeding portions, enrichment, and daily exercise resolves it in many dogs. Persistent coprophagia with weight loss warrants a vet visit to rule out malabsorption.
Disgusting but mostly harmless. Consistent cleanup and supervision resolves it in most dogs. Persistent cases in adult dogs with other symptoms warrant a vet visit.