Juliet is most definitely characterized as being more sensible than Romeo, especially at the beginning of the play, and this character trait seems to be something that she is just naturally born with. Romeo was born to be impulsive and emotionally driven; Juliet was born to be more sensible. However, by the end of the play, desperation leads Juliet to make the same type of irrational, emotionally driven decisions that Romeo makes, leading to both of their deaths.
We especially see that Shakespeare characterizes Juliet as the more sensible character in the famous balcony scene. Juliet shows practicality by being concerned for Romeo's safety should any of her family members find him in the garden. She is so wisely concerned about his welfare that she repeatedly warns him he'll be killed and repeatedly asks how he managed to get there. She further shows sense and wisdom when she resists making any promises of love, warning that making promises "is too rash, too unadvis'd, too sudden" (II.ii.124). She even further sensibly resists Romeo when she makes it well known that she won't have a relationship with him unless he proposes marriage, as we see in her lines, "If that thy bent of love be honourable, / Thy purpose marriage, send me word to-morrow" (149-50). Hence, since Juliet is wise enough to realize that their sudden passions are unwise and wise enough not to let Romeo seduce her unless he is proposing marriage, Juliet is being far more sensible than Romeo and not allowing herself to be carried away by her irrational emotions, unlike Romeo.
However, Juliet does not continue to be mature and sensible throughout the rest of the play. When Juliet's father threatens to disown her should she refuse to marry Paris, Juliet's decided course of action becomes to either commit suicide or follow through with Friar Laurence's plan to fake her death. Considering that the friar's plan was only devised so that he could unite her with Romeo while he is in exile, what would it matter if her father had disowned her or not? Her inheritance would have been useless to her after her faked death. Therefore, Friar Laurence's plan is not the most rational one nor is Juliet's decision to follow through with his plan the most sensible decision. Instead, her decision was brought on by emotional desperation and emotions tend to lead us to make irrational decisions. The better course of action would have been for Friar Laurence to speak to Lord Capulet on Juliet's behalf and explain the situation. Friar Laurence had done nothing illegal or irreverent in marrying the couple in secret. Even if Lord Capulet had still decided to disown Juliet, she would have still been given what she wanted--the freedom to join Romeo in marriage, even during his exile, which would have saved both her life and Romeo's.
Therefore, while Juliet is characterized as more sensible than Romeo in the beginning, she too gets swept away by rash, irrational, emotional decisions, which contribute to her early death.
Anyone who has read the text of the play ''Romeo and Juliet'' can see and note the notable 'development' and 'maturing' of Juiet's character (vis a vis Romeo's and generally, too) n the coure of the story.
It would be a very long exposition explaining why she matures so much but simply, because (a) of having to deal with Romeo's absence and her grief and suffering within, by herself and (b) also by trying to handle and cope with various emergencies and crises by herself.