Weirdly she wasn't actually born into it as guaranteed to be queen. When she was born her uncle David was supposed to become king. When he abdicated the throne to marry Wallace Simpson it changed her life completely. She went from being secondary royalty to being the direct heir to the throne overnight when that decision was made. Also she was a child at the time so as a child she went from being just a wealthy kid who could live as she chooses, to being the future queen and figure head of state with a duty to uphold the constitution of the United Kingdom and its common wealth countries for its peoples. She was the power check for government elected officials, a sort of ceremonial watchdog that upholds the will of the people over all and protects the constitutional rights of her citizens and country.