*to alert us to something
*from fear
*protectively
*from boredom
*through anxiety

If your dog's barking has become a problem, you need to know what triggers it, and why he/she does it before you can address the problem. Each reason would require a different approach.
PLEASE RESIST THE URGE TO GET A CITRONELLA COLLAR or AN ANTI-BARK MUZZLE before knowing what you are dealing with, or you could make matters worse.