Why do so many GSD have mouth open? Panting?

That's a good question, to which I don't have an answer. I have a GSD and he does sit and pant sometimes. I tend to think that he may be hot in the warm house. I usually turn on an overhead fan and open a window a crack in my bedroom and he goes in there to lay down. I really don't know why else he would be panting as he is healthy and only a year old. My older GSD (who I recently had to put down) also panted alot. Maybe it's just common to the breed.