My dog & other dogs poop

Question
My "very handsome" 4 y.o. altered male Cairn has had a proclivity for playing with waste from other dogs since he was a puppy but we have been able to break him of most of this gross behavior. Seamus will go looking for cat poop to eat if we don't watch him but we are consistent about correcting this behavior and have also been pretty successful. However, he has found new pleasure in rubbing and rolling in poop when he finds it. Why is he doing this? He eats premium dog food and gets treats just for being him.He has a big back yard for exercise and is tethered when out in the front yard, unless we are outside then he is untethered. Only dog and has very little socialization with other dogs but he does have neighbor dogs that he "barks at/with/to".
Thanks for your time and attention!
Cynthia

Answer
The terrier is a tenacious hunter, a very large dog in a very small body.  They're smart, stubborn, head strong, sometimes quite dominant, and have a huge prey drive.  Rolling in excrement is both a social behavior and a hunting behavior; this is genetically inherited.  Not all dogs share this behavior but many breeds intended for hunting, herding and guarding do.  Herding dogs roll in the poop of their herd (sheep, cows, etc.) most likely to make it easier to slip among them and do their job of control; the Grey wolf (from whom all dogs are genetically descended) rolls in the scat of other wolves to make a "statement" to his own pack members, to include himself in the pack, etc.  Wolves also roll in the scat of prey animals as a method of disguising their own scent when on the hunt, and they will roll in the remains of a kill to advertise the trophy.  If your dog is rolling in the poop left by other dogs with whom he "converses" by barking back and forth, it's an effort to "arm" himself against unknown threat or possibly communicate as one of them. He is not well socialized to other dogs and may be fearful of them.  Socializing a dog to other dogs, new people and places, is one of the responsibility of dog ownership; the more comfortable a dog is with its surroundings, the happier and more well adjusted the dog will be throughout its lifetime.

 Eating excrement is called coprophagia and is not driven by the same factor as rolling in it.  Most dogs love cat poop, this is a well known fact; the solution is to keep a scrupulously clean litter box (your cat will thank you for it); check the litterbox often and remove excrement immediately, this for the improved health of the cat and to make it impossible for the dog to successfully reward himself.  After a time, when there is no cat poop present and no reward, he should stop the litterbox interest.