Juliet is more practical than Romeo because her whole life depends on the choices that she makes as a woman. One bad choice can ruin her reputation and her future. It was more difficult in Shakespeare's time for women to live in a male-supported and male-dominated world. Back then, marriage was the only foundation upon which a woman could secure her future. Women weren't able to own property or even inherit their own parents' money, so choosing a husband who would show her kindness and respect was very important.
In act 2, scene 2 of the play, Romeo and Juliet get caught up in their emotions and the excitement of the moment. Juliet wants to make sure that Romeo is serious about his feelings for her so that she can secure her reputation and future. She even tells him, "I have not joy in this contract to-night;/It is too rash, too unadvis'd, too sudden,/Too like the lightning, which doth cease to be/Ere one can say it lightens"(II.ii.123-126).
Finally, she tells Romeo that if he isn't serious enough about her to actually marry her then he should leave her in her grief and never show up again; otherwise, they need to get married soon if they plan to continue this relationship. On one hand Juliet is thinking clearly, but then she jumps into a marriage without thinking it through further. So, even though she is a little more practical than Romeo, and tries to make the relationship legal and holy, she acts too quickly, too.