One of my sisters was a "puppy-raiser" for one of the premier service dog organizations in the country. She would get a dog at eight weeks and then raise it for about 18 months, teaching it basic commands and accustoming the dog to being in crowds, riding in elevators, being around other dogs, etc.
Two of her puppies flunked out of service school (but I'm not supposed to say "flunked".....one is supposed to refer to the dog as "released"...okayyyyy). Let's just say if they were human adolescents, they'd both be in reform school. One was adjudged "incorrigible" and the other one was "sullen". She ended up adopting both of them and still has them to this day. The one is still incorrigible and the other one, while I wouldn't exactly call him sullen, is definitely averse to doing anything he doesn't want to do. They're both a couple of big sweeties, though.