My main consideration is having a good bit of savings and a stable, full time job. That way, I don't worry about feeding, training, vetting, grooming the dog, and I can afford daycare or a dog walker if need be.
Then I think about the impact a dog will have on my relationships with BF, family and friends.
I know BF wants to be settled in our house and such before getting a dog, so adding a dog before that is a no-go. He also wants a dog that doesn't shed, or sheds very little, which I prefer as well, so no compromises need to be made there.
I know my family will happily dog sit, but they are not the most dog savvy, so dog would need to be an easy keeper.
I know my friends are fine with dogs, and some would welcome my dog coming over, being a third wheel, etc., so I don't worry about never seeing friends again.
Then I evaluate what I want in a dog, what I don't want in a dog, and if there are any "deal breakers" in terms of behaviours, medical conditions, etc.
Then I go find my perfect dog!