the only thing that comes to mind is maybe the shepard has a dental problem. something like an infected tooth that is giving off an odor? does he eat ok? i can't think of anything else that would make another dog so interested in his front end. usually, it's the back end that gets all the sniffing!

Yes, what Ninapearl said. There have actually been cases of dogs who had habitually sniffed at their owners and the humans found out after the fact that the area the dog wouldn't leave alone was cancerous. So dogs most definitely can pick up aromas not considered normal. Dental problems would pop immediately to mind.

Is it just sniffing or is there any licking going on? It sounds like it could also be over arousal on the part of the spoo. Sometimes they get downright silly and odd when they're so excited, they can't figure out what to do with themselves.

I also would suspect dental problems. But if that checks out fine, I would just say that animals are quirky. I have a cattle dog mix girl who bites the whiskers off of the other dogs. And they let her!

My older female Ginger (13-14 yr old english shepherd) and relatively new 2 yr old male Boo (dobie/setter mix) do this ALLLLLLL the time. She also use to do this with Rusty and Mocha. I read something years ago that its akin to "butt sniffing" and "pee sniffing" - its really dog language for "where have you been and what have you been eating". Remember that dog communicate very much through smell.

In essence... (pun intended) it's "nosiness".