Even in the very first scene in which we meet Juliet, Act 1, Scene 3, she shows a bit of a rebellious streak. It is evident that, as expected by society, she wants to please her parents, but she is also a budding woman who is learning her own mind and places more value on her own desires than anything her parents wish. Therefore, it is not really any surprise when we see Juliet rebelling against her parents in Act 3, Scene 5, after she has married Romeo. The main difference we see is that in Act 3, she is a bit more forceful about her opinion. It seems that the combination of defying her parents through her secret marriage and the tribulations she has undergone over the past 24 hours have matured her into a woman who can find her own voice. Not only that, desperation to prevent sin and remain faithful to her husband also drive her to find her own voice.
Juliet's rebellious streak is especially evident in Act 3, Scene 1 when Lady Capulet tries to persuade Juliet to consider marrying Paris. When Lady Capulet asks her, "[C]an you like of Paris' love?," Juliet's only reply is that she'll "look to like, if looking liking move," meaning that, as her mother suggests, she'll take note of Paris at the ball to see what she thinks of him (I.iii.100-01). However, hidden rebellion can be seen in her next two lines: "But no more deep will I endart mine eye / Than your consent gives strength to make it fly," meaning that since Lady Capulet is only telling her to see if she can like him, whether or not she can like him is all she will consider that night at the ball (102-03). While it is clear here that Juliet is not fully agreeing with her mother, it is also clear that she is trying very hard to please her, just as was expected of children in this era.
However, all desires to please become lost in Juliet's more eminent needs in Act 3, Scene 5. In this scene she is no longer playing the role of the obedient daughter; she has found her own voice as a woman. She refuses to consent to her parents' sudden demand that she marry Paris. Part of her ability to refuse her parents stems from the fact that her rebellious nature has already matured through the fact that she went behind their backs and married a man in secret. However, the grief she has suffered due to Tybalt's death and Romeo's banishment has also matured her from a girl into more of a woman. Finally, the most powerful reason behind Juliet refusing her parents is due to the fact she realizes marrying Paris would be a sin. As a girl brought up in the Catholic Church, Juliet well knows that polygamy would be considered a sin under the Catholic Church. Therefore, Juliet's strong rebellion in this scene is not just based on her preference not to marry Paris, but rather on her religious convictions. We especially see Juliet express her religious convictions in the line, "My husband is on earth, my faith in heaven," showing us that it is her convictions of faith that have also helped her develop into a mature woman with her own voice.