Researchers believe that dog howling is part of a dog's genetics. Dogs are distant relatives of wolves. The urge and ability to howl is in them and they have to exhibit this ability once in a while. Dogs howl to attract attention. They communicate by howling. Dogs howl in response to high pitched sound like a siren. They also howl as a defense mechanism to get away from possible predator. Dogs howl when they are sick or injured.
Howling is normal and natural for a dog. Do not worry if your dog howls once in a while. If the howling becomes chronic or pervasive, you should visit a veterinary doctor.