I was going to mention the same thing. Folks do not often consider the pack instinctive mentality of the family dog. To them, we are simply their "pack". That pack has a hierarchy that they understand completely (at least in their mind). There is an Alpha and they have their place as well. It is ingrained in their instinct, once the bonds are established, to be with, protect and support the pack. We have a terrific lab. For some time, when my son was younger and she was a pup, she had some issues competing for dominance over my son (this was a bit of a problem at times). She played a little rough with him. She is a great watch dog and alerts whenever someone comes on the property. She doesn't like strangers at the door unless me or my wife acknowledges them.
She does have one particular behavior that we have not been able to fully explain. When I (the pack alpha male) is around, she mostly ignores or at least tolerates other dogs. If I am not around, she can be nasty to other dogs when they approach, especially in her yard or with my kids. These are the neighbor's pets I'm talking about. We joke that in our neighborhood, she is the dog bully.
I love a good dog.
She's Beta, second in command. She takes over when you're gone.
I like the other explanation here. But I will add about my previous dog, while paying respect to my submissive girl who just died the other day.
My old Shepherd was a natural dominant dog. Not to be confused with aggression. But she did not “try” anyone as is often suggested. Even as a teenage girl I established who was boss, as well as my parents. She accepted this quite readily albeit stubbornly sometimes trying to ignore my training. But I became boss while she was “beta”. She had no qualms about establishing herself as boss dog over any beast in the area, but she was NEVER a bully or pest of any kind just to prove herself. Only if the dog pushed his luck with her did he get a handful. It was merely attitude, not action. She was very secure in her position.