In thread after thread we hear the problems. Reactive, lunges, barks at everything, hides behind me, hates other dogs, doesn't like people, is not good around kids and then there are the health threads.
I'm sure some do, but I'd be willing to bet a high majority of these type of dogs come from breeders who aren't breeding for anything at all. They probably think they are breeding for a "moderate" dog, but really don't have a clue. Just breeding x purebred to y purebred for cute puppies at $500 a pop.