Because they're scrumptious and delicious. Actually I'm not sure either.

If your dog does that, he's got a poop fetish

Dogs do that because they lack something in their diet. It is not healthy for them as there is bacteria in feces. Think about changing his food. That's disgusting!

Mother dogs do it to their pups its instinctivve. It keeps the den clean. If an older dog is doing it they have a vitamin deficiency. You can get a supplement at Pet Smart to put in their food. That should stop it. Unless of course you dog is just a total pig. Then there is no help.

In the past it was believed that feces eating, also known as coprophagia, was caused by either poor diet or poor health. However, this theory is not supported by current research. "Behavioral research has discounted the idea that it is a dietary deficiency or a pancreatic enzyme deficiency," says Dr. Jo Ann Eurell, a veterinarian and animal behavior specialist retired from the University of Illinois College of Veterinary Medicine. "Dogs are historically scavengers, and this is believed to be a scavenger behavior. "It is important for dog owners to know that this behavior is normal for a mother dog with pups," adds Dr. Eurell. Newborn pups must learn to urinate and defecate. The mother teaches the pups by licking their bottoms. The pups respond to this "tickle" by urinating and defecating. The mother then consumes the pups' excrement, which serves two protective purposes: it keeps the den area clean and it removes smells that could attract a predator. Some pups learn this behavior from their mothers and will stimulate themselves and consume their own feces. Most pups stop by the time they are weaned. It is more difficult to understand why adult dogs eat feces. Some dogs will learn this behavior from other dogs. In some cases, eating feces may be an attention-seeking behavior. For some dogs it is possibly due to anxiety or boredom. Most often the motivation for eating feces is just not known. Some of the above from VetMed http://vetmed.illinois.edu/petcolumns/index.cfm

Lack of nutritions in their diet