Shakespeare creates sympathy for Juliet by showing us her vulnerability. First, she blunders horribly in the initial moments with Romeo when she reveals everything in her heart to Romeo before she has even had time to properly play the courting game. We sympathize because she reminds us of the awkward growing up moments we've all experienced, and because she is so honest about what she's done. (She even says she'll back up and play the game right if he thinks she's too easily won). Second, she still blushes like mad. Once again, this shows her innocence. The more she tries to mask her blushes, the more innocent she appears to us. Finally, we feel sympathetic toward her because her situation is so impossible. Every time the plot intensifies and something new happens, we share a sense of her dismay and sorrow. She is only just in the process of growing up (even though we know she is miles away from maturity)and everything that can go wrong does. Of course we feel for her.
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.