The Prologue to Shakespeare's Romeo and Juliet can be considered the voice of Fate. In Greek and Roman literature, Fate determines the path and end of heroes and heroines. As the Renaissance period reflected back to those traditions, so too did Shakespeare's plays. In this play, Fate has determined that the only way to end the feud between the Montagues and the Capulets is for Romeo and Juliet to die. Whatever choices any character makes to change that fact are to no avail. Therefore, the Prologue for Romeo and Juliet should be considered a voice of warning from Fate.
One warning or lesson might be that pride and fighting lead to consequences that are beyond one's control. Another might be a warning to parents that their actions will be answered upon the heads of their children. Take for example the part that reads: "And the continuance of their parents' rage,/ Which, but their children's end, naught could remove,/. . ."(Lines 10-11). Clearly, Fate is teaching that parents should fix their own problems so that the children don't bear the brunt of them. And if the parents don't take care to satisfy their own quarrels, then Fate will step in and fix the problem by other means. Sometimes those means are the only way to make people listen, too. It seems to take extreme measures to get people's attention when they need to learn a lesson or to stop certain destructive behaviors.