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.

Informing the audience of the ending at the opening of a piece of dramatic narrative is not uncommon. This technique creates anticipation and suspense, very simply, by telling us what to expect. 

This mode of creating anticipation is not unlike dramatic irony. In both cases the audience is provided with knowledge that some (or all) characters do not possess. This puts the audience into a special and often exciting relationship to both the action and the characters of a narrative. 

Hitchcock used his technique often in his films - ex., Rope & Frenzy.