Strangers

• Dogs bark to protect their owners.german shepherd image by Aleksander from Fotolia.com
  
  Dogs are territorial and will protect their owners and home by barking at strangers. They will not stop barking until the stranger has left the territory or until the stranger is accepted into the territory.
  
  

Pain

• Dogs may bark out of frustration or loneliness.dog face image by Albert Lozano from Fotolia.com
  
  When a dog is in pain, it may bark constantly, especially if the pain is emotional, such as feeling lonely.
  
  

Needs

• Dogs might bark if they're hungry.dog biscuits image by Greg Pickens from Fotolia.com
  
  Dogs will bark to communicate a need and until that need is met, they will continue to bark. The need could be hunger or thirst. It could be companionship or to go outside to relieve itself.
  
  

Boredom

• Dogs may not bark constantly if they are kept busy.dog leaping play catch fetch return vizsla weim image by Paul Retherford from Fotolia.com
  
  Dogs need interaction. They are pack animals and therefore need attention. Dogs will often bark continuously when they are bored and want to play or socialize.
  
  

Excitement

• An excited dog will bark and bark but eventually stops.sheep-dog image by petar Ishmeriev from Fotolia.com
  
  Dogs are happy to see their owners come home. They will often wag their tails, jump up and down, and always bark. Once their excitement wanes, so will the barking.