my dog

There are lots of reasons.  

Diet:
What is he fed on?  A lot of the lower quality foods are not very digestible and when they come out they seem just like, well undigested food to the dog.  Changing the diet would very likely help.

Behavioural:
Some dogs do this because they are "weird" about Poo, as a side effect of harsh house training methods.

He likes the taste:
Yeah, GROSS    But only gross to us humans.  It's not gross to a dog! 

You could try adding pineapple to his diet - this should make the poo unpalatable to him.  There are otehr things you can add but I am not sure what they are... maybe others will jump in...

If you are very prompt about picking up after him so he doesn't get CHANCE to eat his poo, then this should break the habit.  But you would have to be VERY vigilant about him not getting a chance to eat poo for a few months. 

double post, sry

.... 

Nothing to add about the why, Chuffy covered the different possibilities.

The other things you can try are adding digestive enzymes to his food (e.g. Prozyme) or products like Forbid or Deter (to give the poop a bad taste). You can get the Forbid and Deter on-line or at your vet. You could also try to "spike" the poop with Cayenne pepper or hot sauce. Do not have your hopes too high though. I have tried them all and nope, they did not work for my dog. Some dogs will respond to one of those things, some won't....

My Callie used to do that when she was about 6 months old.....she would come in with poopy breath and I hated it. She never ate her own poop, but she loved to dine on my other dog's poop before I could get out there and pick it up. They both "go" out in the back yard, which is fenced. When I realized what she was up to, I had to go out with them every single time. To get her to stop, I always brought a few little treats outside with me, and just as she would start sniffing around the poop, I would give her the "leave it" command and reward her. I had to do this consistently for about 2 weeks. She has never done it since. Sometimes I do see her sniffing around his poop, but she looks at me and leaves it now without my having to tell her to.