The training of a Service Dog must result in a dog that is motivated and capable of carrying out essential tasks. Some of the success is based on the skill of the trainer, some in the skill of the client (handler), and some in the character of the dog itself.
A fully-trained Service Dog goes everywhere with the human partner:
To the Grocery Store
To the Doctor
Out for Dinner at fancy (and non-fancy) restaurants
To the movies
etc
The polite service dog will always pay attention to the human partner, help the partner in difficult situations, and stay calm. The Service Dog must not jump up at strangers, "steal" food in stores or restaurants, or growl or interact with other dogs while working.
Selection of the Puppy
To achieve all of this, it is critical that the puppy is selected properly. Simply choosing a suitable breed is not enough: there is so much variation in character even within a litter of puppies that simply picking a breed is not good enough. Temperament is critical, and this is influenced by the history (early experiences) of the dog. Dogs from a rescue organization have unknown history and are often unable to learn to adapt to being a service dog. Dogs from a breeder that doesn't know about the special requirements of the job are also usually unsuitable.