A dog's territory is most important to him.
For him, it can be his house, his own place in the house, where he eats, his sleeping place, even his furniture.
A dog is always possessive about his territory.
Any dog, even if he is very friendly around outsiders, sudden intrusion can make him bark, growl at the stranger like anything.
Sometimes, a dog is overprotective about his territory.
They are related to sounds near his territory and if they find any outsiders involved around, even due to footsteps of the stranger.
They start noting door bell, car or any other vehicle sounds and start barking when they notice any alien sounds coming around their territory.
An overprotective dog might become really aggressive in his behavior.
If he is chained he might not be able to show his aggression clearly except for barking nonstop at the outsider.
In case, if he is walking with the owner, tall and erect, he might be ready to attack the outsider.
It might seem to most owners that they are just protecting their territory, which is quite normal as it should be.
However, this becomes entirely different when your dog is losing control and jumping on your guests to bite them for entering your house.
An over protective dog can be easily noticed by his actions.
Most dogs, when accompanied by their masters, become silent and friendly to outsiders.
At least they stop barking at them.
An over protective dog will never stop barking, until you are out of sight.
They would become alarmed even if they are hearing alien sounds of another car or footsteps around the house.
There have been several cases reported in the past, where dogs attacked their own family members only, who were living outside for a long time.
The dog was not able to recognize them and attacked them when they were coming home.
Try to make your dog familiar with surroundings outside your home, in case, he seems to be overprotective about them.
Do not chain him until it is really required.
Chaining dogs restricts them to a particular area.
They become increasingly frustrated about their territory in restrictions.