Well, hmm. Honestly, I don't think it does.
However, if I had to argue that her actions have this effect, I'd say they have this effect for the following reasons:
1) She's a child. Her parents know what's best for her; she's young, inexperienced, etc. This means she's less likely to make good decisions.
2) Her parents have been searching for a good husband for her for some time. Her love for Romeo is instant. This means it is less balanced, and more likely to go astray.
3) If she agreed with them, love would align with love. (She'd marry who they choose, and everyone would eventually be happy.) Instead, love must fight love, and that's always tragic.