Dogs are pack animals and have a pack mentality. They think that you with your family are other members of their pack! It is important from the beginning and on a continuing basis to establish and maintain a "pecking order" which your dog understands and accepts as his way of life in your family, or his view, his "pack". This requires nothing more than ensuring that your dog knows you are the boss, or "top dog", and he/she is somewhere below you.
Other adults in the household must be above your dog in the "pack". There is only one "top dog". Other adults normally come below you unless of course your spouse or any other adult is actually the "top dog" and you are a little lower in the pecking order.
Children will also be below the "top dog" and above your dog. With younger children your dog is less likely to agree that they can tell him what to do! This is because your dog is well aware that older dogs get to boss young puppies around. To a dog a younger child may well be seen as a puppy. It is funny how dogs know that children are not the same as adults. This is probably because kids tend to be more boisterous, more interested in playing around, often louder, and very often inconsistent in their behaviors. It is important to reinforce to your dog that he/she is in fact at the bottom of the hierarchy of the pack, but with small children, this can be problematic and sometimes even impossible. The dynamic between dog and child is really interesting, and shows us just how innately intelligent our canine friends are.
Back to the dog's pack mentality: If a dog is not taught the core concept of its owner being the top dog, it will be intolerable to live with and very often aggressive or potentially aggressive. This does not make for either a happy family life, or a happy dog. And believe it or not, if a dog is not a happy dog, it is not likely to be as healthy as it could or should be.
Some dog owners think that dog psychology is a bit silly. It is really not. Think about it: Psychological principles can be applied to a lot of animals, certainly most animals that people have as pets. Sometimes dogs can develop problem behaviors for no apparent reason. Those behaviors can most often be traced to an incident which happened to the dog, or an anxiety which has developed maybe due to separation from the owner or any other stressful occurrence. If such problem behaviors surface, your vet or a dog trainer may well be able to suggest strategies for modifying that behavior or psychology. However, the very best way to discourage or prevent such behaviors from surfacing in the first place is to ensure that your dog is happy and contented. Remember: A happy dog is a psychologically healthy dog.
The bottom line is: Make sure your dog always understands his/her place in your family (whether it's just you and your dog, or a family of 10). It is essential for your dog's happiness. If your dog shows any signs of anxiety or starts behaving not quite him/herself, think about what might be bothering him or her. If you can't figure it out, go have a chat with your vet. Simple! You don't want to end up in a position where your dog says: "Humans, if properly trained, make good companions for dogs."