Assistance dog programmes reject a substantial proportion of the animals they begin with, and not because those dogs failed to learn the tasks. The selection targets something else.
What the work actually demands
The task list is the straightforward part. Retrieving objects, opening doors, bracing and alerting are behaviours most dogs can be taught.
The difficulty is performing them reliably in unpredictable public environments, for hours, while ignoring food, other dogs, noise and people attempting to interact.
What is being selected for is therefore emotional stability under sustained novelty, not intelligence or eagerness, which are far more common.
Why breeding programmes exist
Temperament traits relevant to the work are substantially heritable, so programmes maintaining their own breeding lines achieve higher success rates than those sourcing dogs externally.
Records of which parents produced dogs that completed training allow selection across generations on outcomes rather than on appearance.
Retrievers and their crosses dominate not through tradition alone but because these lines combine biddability with a tolerance for handling and a low startle response.
The early environment
Puppies in these programmes receive structured exposure from very early in life, including varied surfaces, sounds, handling and travel.
Most then live with volunteer raisers for around a year, in ordinary homes, which supplies the breadth of experience no kennel can reproduce.
The raiser's job is socialisation and household manners rather than task training, which begins later with professional trainers.
Why so many do not qualify
Dogs are withdrawn for reasons including noise sensitivity, distraction by other animals, over-attachment to one person, and health findings such as joint or eye conditions.
Withdrawal is usually a judgement about the dog's own welfare as much as its performance, since a dog uncomfortable in public would spend its working life stressed.
Withdrawn dogs are generally rehomed, often to their raisers, and many go on to other roles with narrower demands.
Matching a dog to a person
The final stage pairs a qualified dog with an individual, and the considerations include physical size relative to the tasks, pace of movement and household composition.
Handler and dog then train together, because the dog has to transfer its behaviours to a new person and the handler has to learn to direct and maintain them.
The relationship requires ongoing work throughout the dog's career, and programmes typically continue supporting the pair rather than treating placement as the endpoint.