It’s possible to have more than one cause, and thus two different, but equally valid, observations. Both parents who are far too strict, belittling and beating their children, and parents who are too loose, not providing guidance and rules for the children, are bad parents who fail to adequatly raise their children. One of the things they fail is to give the children an adquate sense of self-worth and self-esteem. Children who are beaten think that whatever they do, they fail, so they may turn angy and resort to only beating.
Children who are spoiled or ignored miss opportunities to experience that feeling of accomplishment when finishing a difficult task. They get what they want, so they believe that they have a right to take what they want.
Whenever dealing with extremes, the correct answer is not the opposite extreme, but the middle.
Or, as J.K. Rowling said before her last book came out, when talking about the Dursleys: “there are two abused children in that household” = spoiling Dudley is also a form of child abuse.
I don’t think “ambition” is the right word here. Rather, it’s the short wait time. They learned that a bit of whining will get them what they want. If they were ambitious and ever were given the chance to experience that working hard and taking the right steps toward that goal, that they can reach it. That would be the correct way of channeling ambition.
I wonder how much it has to do also with society in general, not only the parents. If you look around, you see CEOs making ff with milliions even when their companies are crashing, or simply stealing the money; they see states attacking other countries to take what they want; so why not assume that “might makes right” and take what you want by force?
Sadly, this is not completly true. Rather, it seems to be some basic character setting on how to people react to abuse: some people become broken and quiet and sympathetic, and avoid all forms of aggression because they know how bad it feels; other people conclude they can only rely on themselves, and that the only way to get ahead and protect themselves is to hit first.