Why does my dog howl?

From http://pets.webmd.com/dogs/guide/howl...

Howling is one of many forms of vocal communication used by dogs. Dogs howl to attract attention, to make contact with others and to announce their presence.This may indicate they’re lonely, especially if they’re left alone or kept outside for many hours at a time. Some dogs howl to get attention from people. Systematic desensitization and counter-conditioning are two common treatments for fears, anxiety, phobias and aggression. Some dogs also howl in response to high-pitched sounds. If your dog howls in response to some kind of trigger, like another dog howling or a nearby siren, he’ll probably stop when the sound stops. Dogs sometimes howl when they’re hurt or sick.