When you bring in another dog, all you are doing is bringing a competitor into your dog's home.
If you already have a dog and you are a considering another dog, here are a few more things to consider before you start shopping:
1. If you have a dog with behavioral problems, getting a second dog with the idea that the second dog will alleviate your first dog's problems, think again. A second dog rarely does anything but magnify the existing problems.
2. Consider how much space you have in your home. Every animal is entitled to enough physical space, as well as enough time and attention. You should not underestimate how much more time and attention each animal takes away from the others.
3. Does your first dog have problems sharing stuff, especially you, but also bones, toys, or food, with other dogs? Then for the life time of that dog do not bring another dog into you home.
4. Does your first dog get along well with other dogs? If not, do not bring another dog into your home.
5. If you have a cat who has no experience with dogs there is a risk that the cat may never accept the dog. That can create problems for you.
Stay tuned for Part Three: Puppy vs Adult