So I have read up, this is one of the most common problems people have with their dogs but they generally don't like to mention it which is why people don't realise how common it is.
It also states that it is more of a problem where the owners are concerned than it is to the dog. To dogs, eating stools is just another way of getting nutrients and half digested ones at that meaning will be even easier for them to digest.

It can be caused by a wide range of things such as food intolerances or allergies, bordem, apparently, dogs kenneled for long periods often get this, nutrients missing from their food or just out of pure habit. I'm sure many dogs just do it because they can....remember, a dog doesn't have to be hungry or missing in something to want to take advantage of any nutrient source that comes their way.

Pups often do it more because they copy mum when she cleans up after them. Monkey see monkey do sort of thing. They often grow out of this as they get older but as has been mentioned, a lot of dogs do carry on eating stools all their life.

A dry diet tends to produce less palatable stools so thats worth thinking about when trying to stop your dog eating his/her own stools. Meaty diets produce much more attractive stools...well, attractive to a dog, lol.

I read about one dobie who was so good at eating his own stools, he'd spin round whilst he was passing them and catch them in mid air before they even hit the ground, haha!!!! Now that is the king of stool eaters.

Our Jess has always been a stool eater but generally left her own alone being on a complete diet and all, I don't think her own were all that attractive to her. She does sometimes go into "Hunt the stool", mode when out on walks but these stools could belong to dogs fed on meaty diets and such.

It is funny that our Jess also loves rabbit droppings. She'll walk around picking them up for ages.
I've not seen her take any interest in horse stools or anything though. What attracts her to certain stools and not others is anyone's guess.