Its attractiveness to humans. In a wild wolf pack, only the strongest and fiercest wolves survive. But if the small, runty wolves started hanging around the edges of the encampments of early humans, the humans would occasionally throw a scrap of food to a cute-looking pup. You then began to have two parallel lines of natural selection going-- the wolves who stayed wild were being selected for size and fierceness, while the ones who hung around humans were being selected for cuteness and meekness (and later, when humans caught on that dogs were good helpers in hunting, for trainability).