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.
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.