Juliet changes the most in Act 3 as it is in this act when she transforms from a girl holding girlish ideals about love to a woman knowing and understanding what real love truly is. The moment Romeo killed her cousin Tybalt had a great deal to do with Juliet's maturity. Suddenly she is faced with the choice to continue to trust and love Romeo simply because he is her husband or to decide that he is not what he at first seemed to be. Juliet's choice to continue to trust and love Romeo is the moment she learns what real love is, rather than young, immature infatuation. However, while Juliet's greatest character growth can be seen in Act 3, she does change a little from Act 1 to Act 2. Namely, she becomes less of a girl through her embrace of marriage.
When we first meet Juliet, she appears to be still a very young girl. We can see her youth in her apparent lack of interest in boys, which is seen in her resistance towards marriage. We see her resistance when her mother asks her what she thinks of marriage and Juliet replies, "It is an honour that I dream not of," meaning that she has no desires to marry (I.iii.70). However, when she meets Romeo later in this same act, she completely changes her mind about marriage. She feels that she has fallen in love and dearly wants to marry Romeo, as we see in her lines spoken to her nurse, "If he be married, My grave is like to be my wedding bed," which means that she will die a maiden if she can't marry Romeo (I.v.143-44). This transition from dreading the idea of marriage to longing for it shows us that she is beginning to grow from a girl into a woman. We further see her maturity when she actually does marry Romeo in the second act, showing us that the way in which Juliet changes from the first act to the second is by maturing into more of a woman.