One moral in Romeo and Juliet is that it can be disastrous to try to control others' lives.
The Capulets and Montagues held tight to their "ancient grudge," and their children knew that they had to hide their relationship from their parents. Lord Capulet had already made plans for Juliet to marry Paris, and he was unwilling to allow Juliet the freedom to choose someone else. When Juliet attempts to voice her preference not to marry Paris, her father is enraged, telling her to get herself to the church that Thursday "or never after look [him] in the face." Juliet's mother fails to support her daughter's wishes, telling Juliet that she is "done with" her and to "talk not to [her], for [she'll] not speak a word." Lord and Lady Capulet cling so tightly to their own plans that they fail to recognize the desperation in their daughter's pleas, and she is thus compelled to take dangerous measures to try to achieve the life she desires.
Juliet's cousin, Tybalt, also tries to intervene on behalf of the family's "grudge." When he spies Romeo at the Capulet party, Tybalt's sense of retribution grows, and he is determined to fight Romeo to prove that he will enforce the boundaries of hatred between their two families. In the end, this proves to be a fatal mistake, and the scene ends with both Mercutio and Tybalt dead.
In the end, the deaths of Romeo and Juliet bring a peace to the two families, but the tragedy lies in the fact that neither family is willing to consider the desires of their children until it is too late. Thus, the Prince notes that "all are punished" when people are determined to dictate others' lives, particularly when that sense of control grows out of deep hatred.