History goes that Aegon and his dragon conquered the seven kingdoms, which implies that Aegon had good command over his dragon. Jorah said something about the dragon to Dany that dragons can not be fully tamed. Did other Targeryens also have so loose control over their dragons or is it only Dany? The show doesn't give much explanation, does the book says anything over why Dany seems to be unable to control her dragon(s)?