I think that if little dogs are socialized to many humans, all different size dogs (safe larger ones, of course), and kids of all sizes, they become well adjusted dogs that can walk on the ground and not snap at everything they see. If they are carried everywhere, dressed up but not walked like dogs outdoors, and generally treated as if they own your lap, your bed, and the crook of your arm, they will become nasty or fearful little PITA that are not a pleasure to live with at all. Socialization, training, exercise - just like the big dogs...