It is a natural instinct for your pup to howl. You may find that your dog will howl in response to the phone ringing or sirens. Dogs are wired to howl like this as this is what wolves do to signal to their pack members where they are.
A beautiful young Great Dane howling outside
Dogs suffering from separation anxiety tend to howl when they are on their own. Some dogs howl in pain, or when they are nervous or anxious.