Why do you think the Nurse wants to help Romeo and Juliet?

Juliet is quite young in this story. And, the nurse has raised her from an infant. The nurse probably knows Juliet better than her own parents as the custom was to have babies and then give them to wet-nurses to be raised until they were old enough to participate in the family business.  Since Juliet is a girl, the only thing she can do is marry well to someone that will help the Capulets prosper.  We are not told how old Paris is, but we can assume that he is quite a bit older than Juliet and someone her father knows well.  This is not necessarily going to make a good match for Juliet, and the nurse knows Juliet's heart is taken by Romeo.

In the end, the nurse caves in under pressure from the family and in Romeo's absence, she tells Juliet to marry Paris.  I think the Nurse had hopes that the Montague and Capulet families would be on friendly terms once the children's marriage was made public, but it was done in secret and without the proper banns being said. It could be argued that the marriage was not legitimate.  And, in Romeo's absence, he is useless to Juliet. It would be better for her to be married quickly to Paris just in case she had conceived during her intimate night with Romeo.

It is also possible that the nurse was using the entire situation to "get at" the Capulets for any possible mistreatment of her during her employment with them.