After the announcement of the marriage arranged between Juliet and Paris the Nurse sides with the Capulets and feels Juliet should marry Paris.  Juliet is very devastated and hurt by the nurse's reply.  She is offended because Juliet felt that the Nurse was her confidant and "Mother."  At the end of Act III, Scene 5, Juliet calls the Nurse "a wicked fiend." I feel that the audience felt that she was cold, heartless and betrayed Juliet.

The nurse has always been Juliet's cohort in setting up secret meetings and running interceptions between Romeo, Friar Laurence, and even going behind Juliet's parents backs. She has felt that Romeo was "an honest gentleman, and a courteous, and kind, and handsome,.."(Act II Scene 5)
When the nurse learns that he has killed Tybalt, "the best friend I ever had," she quickly changes her opinion and decides that Romeo's banishment is like death and it is a final decision. Lord Capulet has also changed his mind about the marriage. On one hand he explains to Paris that his proposition has not been discussed with Juliet, due to Tybalt's death, but on the other hand he exerts his parental dominance and tells Lady Capulet to go tell Juliet she is marrying Paris on Thursday. Lord Capulet shows no emotion for the situation and feels that life should go on as always.

Juliet refuses to marry Paris and is confronted by her raging father who totally discounts her feelings and threatens disownment if she doesn't do what he says. When Juliet is by her parents, she turns to Nurse, who has been her "mother" her whole life only to find that she is no longer her ally, but her opponent. The nurse tells her that Paris is by far more attractive than Romeo, and there is no comparison between the two. Her 2nd marriage would be better and she should do what her parents want. Romeo is gone and he can't come back so he might as well be dead. It is hard to see the nurse turn on her "child", but the nurse is probably afraid that someone will find out about her interventions with Romeo and Juliet and their secret marriage. It is easy to see why Juliet would feel betrayed by the only person she has ever confided in. Juliet now feels desperately alone and hopeless.

The nurse has been a supporter of the relationship between Romeo and Juliet until Act III, Scene 5 when Juliet's marriage to Paris is announced.

After being threatened with being disowned by her parents if she does not marry Paris, she turns to the nurse and asks for her help. The nurse responds that she should go ahead and marry Paris because Romeo has been banished and as good as dead.

Nurse:
Nurse.
Faith, here 'tis; 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.

This change in the nurse forces Juliet to go forward alone in her quest to be with Romeo. The change in the nurse makes the reader question her integrity and true concern for Juliet. Rather than supporting Juliet, the nurse has dismissed Juliet's true feelings.