I have a theory on the prologue. I think Shakespeare wanted to tell us that things were going to end in tragedy so that we would be prepared, and we would know what was going to happen and therefore follow his message a little better.
Shakespeare prepares us for the story.
From forth the fatal loins of these two foes(5)
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife. (Act 1 Prologue)
There is no mystery that Romeo and Juliet are doomed. We can’t wait to find out HOW they are doomed though. Since we are not shocked by that, we can take full advantage of the foreshadowing and follow their destruction from beginning to end. It is clear that the two families need to end the feud. We are anxious to find out how.