"Be not afraid of greatness: Some are born great, some achieve greatness,
and some have greatness thrust upon them." William Shakespeare
We should not be afraid of greatness in any way. Greatness is a positive
thing. "Some are born into greatness" means some people are born into
families who are known for their greatness: Some of the royal families,
children of Kings, Queens etc.. Here in the U.S., the Kennedy family for
example. "Some achieve greatness" means some people contribute the
world with inventions, being a wonderful leader, athletes, entertainers,
a famous director like Steven Spielberg are examples of those who achieve
greatness.
"And some have greatness thrust upon them" means greatness comes to
them for something they had done by accident or without wanting the
greatness.