Some literary critics have argued that the Nurse is deceptive and really merely motivated by her own personal gain. Martin Stevens is one such critic, and even points out that she accepts payment from Romeo for going to him the morning after the ball to inquire of his plans for marriage ("Juliet's Nurse: Love's Herald"). Romeo also pays her for her secrecy. Though she protests at first, saying, "No, truly, sir; not a penny," when Romeo insists a second time, she protests no longer and agrees to have Juliet sent to Friar Laurence's cell that afternoon (II.iv.170). From this, as well as the fact that Nurse so quickly changes camps towards the end of the play and advices Juliet to forget about Romeo and marry Paris, Stevens, along with other critics, argues that the Nurse is motivated by her own gain. However, that begs the question, what would Nurse have to gain from Juliet's marriage to Paris? As Juliet's childhood Nurse, she would not likely remain in Juliet's employment once Juliet is married and, therefore, would have nothing personal to gain.
Instead, it seems perfectly reasonable to argue that Nurse is motivated out of her love for Juliet and wants what will make her happiest. Hence, when Romeo proves to be questionable through his actions, Nurse doesn't hesitate to change sides and believe that Paris would make her happiest. It's very clear from the first scene in which we meet the Nurse that she loves Juliet as her own daughter. In fact, her own daughter who was close to Juliet in age died, which enabled Nurse to become Juliet's wet nurse. Nurse is also very happy with the prospect of Juliet marrying Paris and praises his good looks and manners, calling him a "very flower" of a man (I.iii.82). When she returns from consulting with Romeo, even though we don't know how serious Nurse is because she next praises Romeo's looks, she tells Juliet that she does "not know how to choose a man" (II.v.). If Nurse has her doubts about Romeo from the start, then it makes perfect sense that she is so quick to dislike him the moment she learns he has killed Tybalt. Also, when Juliet convinces herself to think ill of her own husband and begins morning his loss due to banishment, it makes perfect sense that Nurse would agree to go find him and bring him to Juliet as, due to her love for Juliet, she wouldn't be able to bear to see her grieve. Finally, since she distrusts Romeo, it also makes perfect sense that she would take the opportunity to promote Paris above Romeo, saying that Paris is the better man. Nurse has Juliet's best interests and happiness in mind when she states that Paris is the better man.
Hence, while some critics argue that she is motivated purely by gain, it seems to make more sense to say that she is motivated purely by her love for Juliet.
The short answer is motherly love, but that is too simple.
The real question is, "Who is Juliet's real mother"? Lady Capulet, like most noble ladies of the time, had little to do with the raising of her own child. She does love Juliet, but does not know Juliet as the Nurse does. Lady Capulet looks to the prosperity of the family as a whole, while the Nurse is more interested in Juliet's personal happiness.
One can also consider the earthiness of the Nurse. She is sexually experienced and alludes to it being pleasurable. She wants Juliet to be able to know that pleasure in a loving relationship, as opposed to a simply political one. Again, this is an accurate reflection of the times; the wealthy and noble married for more wealth and power. Paris is an ideal suitor in this way, but Romeo is about love.