Why do dogs bark
 Dog Barking and its meaning.

• Continuous and fast barking, at a medium pitch: Alert. Problems. Some is entering our territory.
• Continuous and slow barking, at a low pitch: The intruder or danger is close. Prepared to defend itself.
• Fast barking with pauses every 3 or 4: Warning of a problem approaching. Asking you to investigate.
• Long and drawn-out barks at a high pitch, with pauses between each one: I'm alone and need company.
• One or two short high pitched barks: The most normal greeting.
• One bark normal pitch: Curious, alert.
• Short back in a high pitch: Shows surprise. If it's repeated twice it means "Look at this!" If it's longer then it calling. Many dogs use this when they want to go out.  
• Brief bark, at medium pitch: Happiness.
• Faltering bark at medium pitch: Asking to play.
• Howl or short bark at a high pitch: "Ouch!" Response to sudden pain.
• Repeated howls and regular intervals: Suffering from extreme pain or something that scares them. 
• High pitch or urgent barks that sound desperate, without apparent reason: It is a way some dogs use to let out steam.