Anyone who has ever read anything about dogs and dog behavior should know dogs are pack animals much like the wolves they are descended from. Unfortunately, when they bring a new dog or puppy into their home, they elevate the dog to leader status and then do not understand why the dog won't listen.
Another unfortunate misunderstanding is believing that because the resident dog was there first, the new dog will not be the leader dog. This causes many animals to be returned to shelters because two or three weeks after the new dog is in the home, it begins fighting with the resident dog. Most of the time the fights sound much worse than they are, and they begin when the new dog or sometimes the resident dog, are trying to establish their dominance.
Junior, who weighs in at about 127 pounds has always been the low dog on the totem pole in my house. As a puppy, he was raised with a dominant female and has always let the women push him around. He really just doesn't like to fight.
When I introduced him to Gypsy, who weighed about 75 pounds then, his little nub tail could not stop wagging. The first week you would not have known these two dogs had not lived their whole lives together. Junior was first and Gypsy was low in the pack.
I happened to be on the phone the first scrap they got into and it was over food. Gypsy tried to actually take food out of Junior's mouth. Luckily, the person I was on the phone with is an animal behaviorist and trainer and she asked me was there any blood. I told her no. Her response was, let them duke it out. It happened twice more, never any blood, and now Gypsy has risen to the top of the totem pole over Junior. He doesn't really mind I don't think, but they had to work it out among themselves. I am the leader and they come under me.
Many people would have returned Gypsy to the shelter saying she was picking on Junior and since Junior was there first, he should be higher than her. I see this all the time. The truth is, I could not make Junior top dog. He had to retain that himself, and he was willing to back down and let Gypsy take that spot.
To anyone who is thinking about bringing home a second dog please remember that you cannot orchestrate the heirarchy within the dogs. They must work it out for themselves.