The moral emphasis in Romeo and Juliet is on not letting emotions rule a person's life.
Both the hatred between the rival families and the emotional conflict the main characters had to deal with are a representation of that. On the one hand, it questions whether it is right to hate someone simply based on something as superficial as their name, without taking the individual into account. On the other, it shows the moral dilemmas Romeo and Juliet have to consider; their intense emotions for one another, or doing the right thing by their families.