What is my dog thinking when I get him howling?

This is pure curiosity. A long time ago I discovered I can get my dog, Capone, to howl with me. Initially I think I was singing (which can only be likened to the sound of a dieing moose) and he cocked his head back with a howl. Since this discovery I’ve had fun getting him to howl with me every once in a while.

I know howling is a form of long distance communication for dogs, but still I can’t help but wonder what he thinks is happening when I do this. Drop some knowledge on me Fluther.