Dogs howl to gain attention, announce their presence, send a warning, or to express anxiety. Howling is a way that dogs and wolves to communicate with one another over long distances.
When dogs howl at sirens or other high pitched noises, it’s often because they are responding to what they believe is another dog's howl. In nature howling is a two way communication mechanism.
Dogs will also howl if they are experiencing separation anxiety. In this scenario, their howls are intended to call members of their pack. For example, if a dog is left home alone, the dog may howl in an attempt to re-establish contact with his owner. Howls work better for long distance contact then barking, as howling tends to travel further.
Howling also allows dogs to express strong emotions, including fear, anxiety or aggression. Sometimes howling can simply indicate that a dog is excited about something, as howling it was often a group bonding exercise wolves used to prepare themselves for a hunt.
In order to determine the cause of a dog's howl, observe all of the dog's other behaviors and look for signs of anxiety and excitement, such as pacing or walking back and forth nervously.