In nature, dogs bark to raise an alarm at the first signs of possible danger or to herald a new arrival. Barking is an important means of canine communication. According Cesar Millan, there are generally four basic triggers - and each carries a very clear message.
Scared, confused, spoiled, and boredom are the most common triggers. The best thing you, as a dog owner, can do is to identify why the excessive barking is happening and then work toward fixing the problem.