He ate another dogs poop!

Coprophagy (eating poo) occurs in many more species than people are aware of. Not only is your dog getting bacterial flora from eating feces but there are also a number of nutrients that are made by the bacteria in the colon which dogs (and most other species) can not absorb. Therefore eating fecal matter allows them to get more out of the food that they eat.

In the first place, the previous dog's owner should have cleared up after him. In most places it's an offence not to do this! It is in a dog's nature to occasionally eat other dog's poop. Puppies do this by instinct and the reason is that they need to populate their stomachs with the bacteria that help them digest their food. Humans also need 'stomach flora' and you may have noticed that, if you have been on a particular course of antibiotics, your stomach has been upset for a day or so. We tend to get our stomach flora naturally from the variety of things we eat; dogs tend to only eat meat (in the wild) or factory-made dog food, which may be nutritionally correct but is sterile. It is quite possible that, if your dog has recently had a course of antibiotics, his body was just telling him to correct the balance. It's also possible that he was exhibiting 'puppy behaviour' because many domestic animals have actually been bred to retain puppy-like behaviour - they never completely 'grow up' because we like them that way! Yes, it's gross to us because normal humans are brought up to think poop is exceptionally dirty. It is 'learned behaviour' (and, no doubt, of value to us) and we aquire it from our parents at quite a tender age (around a year old). Dogs aren't humans, however, and their mothers teach them all sorts of things but not that!