At first, the Nurse, who is Juliet's confidant and knows all about her relationship with Romeo, supports the young couple and advises Juliet to not listen to her parents and to listen to her heart instead. After speaking with Romeo, the Nurse is certain that he loves Juliet as well, and she suggests that they marry in secret.
Then hie you hence to Friar Lawrence's cell.
There stays a husband to make you a wife.
In act 3, scene 5, Juliet is upset after her fight with her father, who is very angry at her for not accepting the arranged marriage with Paris. She then turns to her mother and tries to convince her to speak with her father and change his mind and hopefully stop the marriage from happening or at least postpone it; she even says that she'll kill herself if she has to marry Paris. But her mother refuses to help her and insists that Juliet listen to her father.
In desperate need for some comfort and sympathy, Juliet, distressed and anxious, goes to the Nurse to tell her about the fight with her parents and to ask for advice. Contrary to her previous advices, the Nurse tells Juliet to forget about her young and immature lover and to marry Paris instead; in her opinion, Paris is a gentleman and a much better match for Juliet than Romeo, who could never do anything for Juliet now, as he's banished.
Romeo is banished, and all the world to nothing
That he dares ne'er come back to challenge you,
Or, if he do, it needs must be by stealth.
Then, since the case so stands as now it doth,
I think it best you married with the County.
O, he's a lovely gentleman!
Romeo's a dishclout to him. An eagle, madam,
Hath not so green, so quick, so fair an eye
As Paris hath. Beshrew my very heart,
I think you are happy in this second match,
For it excels your first, or, if it did not,
Your first is dead, or 'twere as good he were
As living here and you no use of him.
Juliet, who's used to the Nurse always supporting her and always being there for her and on her side, is so shocked, confused, and even offended that she promises to never listen to the Nurse again and stops being her friend after that.