Why do dogs howl? Howling is an instinctive behaviour and is a form of communication, thought to call together members of the pack or the familiar people known to the dog.
Like any genetically hard-wired behaviour, it can be triggered by various things. Loneliness or boredom can be clear reasons why your dog howls, but dogs may be triggered by similar sounds such as certain music, sirens, or even people mimicking a howl.
Excitement or anxiety can also prompt your dog to howl, and howling (often coupled with several short barks) is a common symptom of separation anxiety, when a dog is unable to cope with being left home alone.
If this applies to your dog, you can work on giving your dog something to do aside from howling when alone; a tasty stuffed Kong can be a positive alternative.