One good way to determine a writer's message, or theme, is to look at how characters change over the course of a story. Romeo and Juliet themselves don't change much, they are impetuous youngsters all the way through. But at the end, as a result of the tragic events, their families, the Capulets and the Montague's, vow to end their feud.
So the theme, which could be stated in many possible ways, should have something to do with the fact that sometimes it takes a significant tragedy to overcome a problem (the feud). You could also focus on the idea that parents and families can be blind to their children's needs when the are distracted by worldly concerns.