Our dog was eating poop.  I explained to my vet that we've tried every kind of diet with our dog.  Expensive, cheap, dry, moist, consistent, rotating, etc.  The dog would still eat poop (more often than "rarely").

In my most professional voice, I asked "Why do dogs eat their own feces?".  The doctor took his glasses off, leaned in and said "Well sir, dogs are gross."