I think that Juliet's primary mood shift in Act III, sc. 5 is one from devoted daughter, hurt at her parents' lack of embrace of her will, to one of defiant daughter, willing to do what is needed to realize her own desires. In some respects, I think that the mood shift is indicative of Juliet's change from girl to woman. The scene is one in which Juliet starts out as relatively sad at her parents' request. Yet, the forceful nature of her parents' response is the bridge at where the shift in her character is most demonstrative. Both Lady and Lord Capulet do not speak with anything in way of tolerance, understanding, or even pragmatic flexibility. Their language is one of absolutes, definite statements filled with images of ultimatums of capitulation or exclusion. Certainly, I think that Juliet absorbs this and understands within such a climate of emotion between she and her parents, there is absolutely no way there can be negotiation between what she wants and what her parents' demands are. It is here where I think that Juliet's mood and her temperament shift in this scene from the idea of trying to be an obedient daughter to her parents' wishes to being a woman who has to stand on her own for the man she loves.