If it hadn't been for the power of love, it's more than likely that Romeo and Juliet would hate each other, just as successive generations of Montagues and Capulets have hated each other. Their feuding families have been at each other's throats since time immemorial. In fact, their feud has been going on for so long that no one even knows how it all started or why.
But because Romeo and Juliet have been struck by cupid's arrow, none of that matters to the star-cross'd lovers. Though both are aware of just how dangerous their relationship is in the context of Veronese society, they are so deeply in love with one another that they're prepared to take enormous risks in order to be together. Normally loyal and devoted to their families, Romeo and Juliet are so in thrall to the power of love that they defy their families' wishes and pursue their passionate relationship.
The power of love can certainly move mountains, as they say, and here it presents a fundamental challenge to the prevailing social order, which is built, among other things, on the absolute sovereignty of parents when it comes to their children. Parental power is so strong in this society that there's only one other power capable of breaking it, and that's the power of love.