Complex sentence: Dogs growl as a form of communication, most often when they are angry or showing signs of aggression, however can also growl when they are playing with other canines/humans, growling over their possessions, are in pain, or during territorial displays.
Split into simpler sentences: Dogs growl as a form of communication, most often when they are angry or showing signs of aggression. Dogs can also growl when they are playing with other canines/humans, growling over their possessions, are in pain, or during territorial displays.