Howling is a non-specific behavior. Howling can occur when a dog is distressed, feeling territorial, stressed in a situation that they cannot get out of, or responding to persistent noises such as the sound of a siren. Finally, I imagine it is a fun activity for some dogs—kind of like singing in the shower. —Dr. Ciribassi
Wolves primarily howl to alert an enemy pack they’re ready to rumble or guide a lost member home. Dogs howling when you leave may be an attempt to get you to return. Howling at other dogs may signal, “Get lost!” or “I’m over here!” —Dr. Ward