First and foremost, a guide dog should have rock-solid nerves. He should be calm and confident, obeying even in the midst of chaos. He should not be easily frightened but also should not be ignorant of real danger when it presents itself. Just as you wouldn't want a guide dog who trembled at the sight of a passing car, you would not want one who stood happily in the road as one speed straight toward it. In other words, you want a dog with an abundance of good common sense.
He should be biddable, which means he should have a desire to please his master and to work as a team member, choosing to perform his job out of loyalty even when it is unpleasant and he'd rather be doing something else (like staying home warm in bed instead of out on the streets in the sleet taking his master to the pharmacy for essential medication).
He should be intelligent and trainable. He should be an excellent problem-solver because it is impossible to predict every possible puzzle a dog might encounter in his working life and he must be able to apply what he knows creatively in new situations to make safe and reasonable decisions.
The ability to exhibit "intelligent disobedience" is also prized. A guide dog intelligently disobeys a command to go forward when it would put his master in danger, such as when a car is coming. When the dog refuses the command, it falls to the owner to determine why and then make an informed decision on whether to proceed anyway, wait, or take a different path.
Since the typical guide dog doesn't begin his working life until he is nearly two years old, and he requires very careful rearing and training costing typically $20,000 to $30,000, a good candidate for guide training must be young enough and healthy enough to have a long working life. Guide dog candidates are screened for health issues such as hip dysplasia before they begin formal training.
Guide dogs should also be of an appropriate size: large enough to work in a guide harness (with its ridged handle that signals the owner) yet small enough to fit in small spaces under chairs and tables. Most guide dogs are German Shepherds, Labrador Retrievers, or Golden Retrievers.