Dog ownership is rewarding, fulfilling, and down right fun! When you love your dog so much, it makes sense to want to add another to your family--a new playmate, a friend, another soul to love--shoot, I have 4 (and a cat, but he thinks he is a dog! ;)
I want to bring awareness to something that may get overlooked in the honeymoon phase of adopting/buying/fostering a new dog or puppy. YOUR OLD DOG! I'm not implying that you will forget about your first dog; on the contrary, I think you will be very excited for him/her! What I actually want you to think about are your dog's behaviors in your life right now. Are you happy with how your dog behaves? Does your dog respect you and your guests? Do you feel like you have good control of your dog when you are out in public? Does your dog bark non-stop out of the window, or fence fight with the dog next door? Does he pull you down the street on the leash, barking and jumping at people and other dogs? Does he potty in the house or destroy things when you are gone?
If you are struggling with your dog's behaviors, it is VERY likely that your new dog will begin to mimic these behaviors--the good and the bad. Adding another dog to the group may seem like a good idea, for instance if you feel your dog is "bored and needs a playmate" because of destructive or hyper tendencies, but you must understand that, from puppies to older dogs, they learn from each other and will learn good and bad habits. If barking aggressively at people, wildlife, and cars going by is a fun activity your dog likes to do now, it's almost guaranteed you'll have 2 dogs doing that in a short amount of time. They are pack animals after all!
I want to encourage you to look at your relationship with your dog now and figure out if you are the pack leader. It's important to know that you are in control, you're happy with your dog's behaviors, and you know your dog represents a good role model for a new addition. If you have a dog with questionable tendencies or down-right bad behaviors, I'd highly recommend doing some training and leadership building skills with your original dog before adding #2 (or #3, #4, etc.) to your pack. It's much easier to have a well-behaved dog help teach your new dog good habits (like looking to you for guidance!) than to have 2 dogs who are out of control because they have no structure or leader. Plus, you'll already have a well trained dog, so you'll know what to do to create a calm and peaceful household with another because you've done it once already :) Remember, the people you hang out with influence you--the same can be said for our dogs.