Juliet’s nurse agrees to help her secretly marry Romeo because she cares about Juliet and wants her to be happy, and she knows she loves Romeo.
Nurse tries to explain to Juliet that she is better off with Paris, because Paris is a gentleman. Juliet refuses, though, and Nurse wants to do whatever Juliet wants. She has raised Juliet from a baby, and since she has no children of her own she looks upon Juliet as her own.
Nurse explains to Romeo that she does not approve of him, but Juliet will not be disuaded.
I anger her sometimes, and tell her
that Paris is the properer man; but I'll warrant you, when(190)
I say so, she looks as pale as any clout in the versal
world. (Act 2, Scene 4)
Therefore Nurse agrees to help Juliet get to Romeo. She passes on the message, after some teasing, and makes sure that Juliet has an excuse to visit Friar Lawrence, so she can marry her Romeo in secret. She makes sure to bring Romeo to Juliet, and tells them when it is time to go in the morning.
Despite her bawdy ways, Nurse really does care about Juliet. She tries to look out for her interests in a way that her own parents don't. She wants Juliet to be happy and secure. She finally decides that helping Juliet get what she wants is more important than helping Juliet do what is expected of her.