What motivates the Nurse in Shakespeare's Romeo and Juliet?I think it might be Juliet because she does so much for her...but I am not sure.

During the Renaissance, the role of nursemaid to the
aristocratic children was often played by poor maiden relatives taken in by
their family.  In such a position, the Nurse has come to love Juliet as a daughter and
is very proud of her.  When Romeo approaches her in Act I and asks who Juliet's mother
is, the Nurse claims that role practically for
herself: 

Marry, bachelor,
Her mother
is the lady of the house
And a good lady, and a wise and virtuous.

I nurs'd her daughter that you talk'd withal.
I tell you, he that
can lay hold of her
Shall have the chinks.
(1.5.119-124)) 

In keeping with her
close bond with Juliet, the Nurse arranges Juliet's and Romeo's nuptial night together
before he is banished.  Thus, she is willing to betray her employer in order to ensure
Juliet's happiness.  It is only after Romeo's banishment that the Nurse fails in her
loyalties as she counsels Juliet to renounce her marriage to Romeo and marry
Paris.  However, she still has Juliet's happiness in mind, believing that Romeo will
never return. 

However, prior to this, when Lord Capulet
insists that Juliet marry Paris, the Nurse does come to her
defense:

God
in heaven, bless her!

You are to blame to rate her so.
(3.5.173)

That the Nurse
loves Juliet dearly is evidenced in the final scene of Act IV as the Nurse is reduced in
grief to a parody of herself as she tries to waken Juliet, speaking nonsensically about
sex:

Go, you cot-quean, go,
Get you to
bed! Faith, you'll be sick to-morrow
For this night's watching.
(4.4.6-8)