Poor Juliet! She is so misguided, so young, and so vulnerable that the older one gets, the more empathy audiences and readers have for her plight.
First, consider Juliet's age. Most scholars peg her at the tender age of thirteen or fourteen. The reason for this timeline is that her father is eager to marry her off as quickly as possible and the early teens were the acceptable age to do so. Like all adolscents, Juliet suffers from the blush of hormones and what might today be called "puppy love." She does not have the ability mentally, even if she barely does physically, to make an intelligent choice.
Her mother loves her, but is aloof. It is really her nurse who has raised the girl, from her infancy to her untimely death. Though she too loves her charge, the nurse is of lower class and uneducated. Futhermore, Juliet has the nurse pretty well wrapped around her little finger. Not so good from a guidance perspective.
Friar Lawrence, who should have known better, fails Juliet too. He should have talked the young girl out of such a ridiculous scheme, or at least informed someone who could stop the tragedy from occuring.