In the past, Nurse has always supported Juliet and been happy to see her do whatever makes her happy. Nurse even accepted Juliet's secret marriage to Romeo and did whatever she could to help the couple; she carried messages back and forth between the two when they were attempting to arrange a meeting time and place.
However, Juliet's failure to agree to marry County Paris, which her father demands (he was not aware of her secret marriage), is not approved of by Nurse. Nurse reminds Juliet that Romeo is banned and states that the chances of him returning to her are virtually nonexistant; she goes on to praise the virtues of Count Paris and to state that Romeo is dead, or as good as dead, to Juliet. When Juliet questions her feelings, Nurse says that they come from her heart and soul.
Nurse's belief that Romeo will not return to Juliet and that it is best for Juliet to marry Paris cause a rift in the very strong relationship that the two have shared since Juliet was a baby. Juliet tells herself that she will never trust the nurse, who has been her best friend, again due to the Nurse's betrayal. Juliet is also angry and hurt that Nurse acts in such a "two-faced" way; although Nurse complimented and bragged on Romeo at one point, she now has nothing positive to say about him.
Ancient damnation! O most wicked fiend!
Is it more sin to wish me thus forsworn,
Or to dispraise my lord with that same tongue
Which she hath praised him with above compare
So many thousand times? Go, counselor!
Thou and my bosom henceforth shall be twain...