Pack Leader

     As a dog owner you may here these words  "Pack Leader" a lot if you are a  fan of dog shows.
Well personally I believe in these words, I believe in your household there must be a pack leader in the house. I believe you have to establish pack leader skills by showing the dog you are in charge because if not the dog will become leader over you.
     The way a dog shows leadership skills is by showing it dominance. Which can be shown in a variety of forms. The dog could be over protective, not listening to you (but this could also mean it does not know its name which then it could be taught), by showing aggression, and can be pushy. Every dominant dog has its own way of showing its dominance. It also depends on your situation, an example would be if a dog was over protective of his female owner when her boyfriend would come over. A dominant dog can be difficult to deal with and might enforce its leadership with its teeth. This behavior can be corrected, but I recommend a expert's advice.

This is dominant behavior.
The corgi is being above the other dog. While the other dog is being submissive.

A dominant dog who is not pack leader can be harmful to the pack, because he/she may doing something unexpected. Which can lead to problems in the whole pack spreading the aggression.  

If your are unsure about your dog I would consult a professional.