In Act III, Scene 5, Romeo needs to leave Juliet or he will be killed if found by Prince Escalus's watchmen. Juliet is very insecure that it will be forever before she sees Romeo again and she needs reassurance that he will be with her.
As befitting her youth, Juliet is impulsive and a starry-eyed romantic. She has been raised by her nurse, a fact that distances her somewhat from her mother (she listens and confides in the nurse when she has a problem, not her mother.) She loves her father, but he is also not a great example of parenting. He plans Juliet's marriage, claiming that she can marry whom she likes, but only to someone he consents to; his claims of her freedom of choice are pretty paltry, however, as he has already selected a mate for her and has no doubts that she'll agree.
Juliet, therefore, has the age-old feelings of teenage rebellion. She wants her freedom; freedom to escape her parent's home and freedom to fall in love. Romeo is attractive to her because he is forbidden; he is the son of the family's long-time enemies, the Montagues, a fact that makes him all the more attractive.
Juliet lacks the maturity, however, to face her parents and say directly what it is that she wants. Instead, she and her equally immature lover (shortly to be husband) act clandestinely to pursue their own goals.
When Romeo leaves, Juliet is bereft. She will stop at nothing to be reunited with Romeo, enlisting everyone she thinks she can trust to make this happen (chiefly her nurse and Friar Lawrence). However, she is due to be forced to marry Parris. If she does, she will be a bigamist. Her love and her duty, thus, come sharply into contention.