What does this mean “when you were young and your heart was an open book”

After hamlet's death the rest seems superfluous. why is the rest important

Which of the following will most likely happen if the pancreas does not produce sufficient insulin? blood sugar will decrease because insulin lowers blood sugar when it rises too high. blood sugar will increase because insulin increases blood sugar when it falls too low. blood sugar will increase because insulin lowers blood sugar when it rises too high. blood sugar will decrease because insulin increases blood sugar when it falls too low.

My friends asked me, “are you not coming for the party? "change into passive voice