The crux of the tragedy in Romeo and Juliet is the absurd nature of revenge and the destruction it brings. The Montague-Capulet feud had been going on for so long that nobody could even remember the original grievance! From the very onset (line 79), the Prince invokes the two families to peace and warns them of the bloodshed which will follow if they 'keep going at it.' But of course they won't listen, and everybody knows what happens after that! This is the "I told you so" precursor to a tragedy which could so well have been avoided had the families listened to the Prince's warning and had they taken his advice.
Then comes the unravelling of the 'woeful tale.' Note that before Romeo and Juliet meet, they are each involved with somebody else. Paris is lurking around in the background waiting for Juliet to grow up while Romeo is hopelessly infatuated with Rosaline. These two prototypes representing superficial eros sharply contrast with the 'one true love' ideal that so incarnate Romeo and Juliet once they meet.
Note that the story line is first chaotic, then runs along with increasing speed until its tragic end. After panning around to catch the global scene, the story line then hones quickly in on Romeo and Juliet, victims of prejudice and vengeance.
There is an element of the Cassandra syndrome here; because truth will not be heard, the path of destruction runs its natural course. This heightens the sense of hopelessness and enhances the final tragic effect.
In a sense, Shakespeare does introduce Romeo and Juliet in the Prologue, when he presents the idea that 'a pair of star-cross'd lovers take their life' over the course of the next two hours. The fact that he does not name them here is the work of a dramatic master.....he has already set up the situation; we know that their families are feuding and have been for a very long time ('an ancient grudge - the implication being that no one really remembers what it was about). Shakespeare also tells us that in the space of two hours, both will be dead. How exciting is that for an audience? We now know what is going to happen in the dramatic world of the play, but not how it is going to happen. He continues to delay the entrance of both characters by showing us the day to day effects of the feud on the kinsmen of the houses of Capulet and Montague, but we still do not see the title characters themselves. By the time they are finally introduced, we see that they are not epic characters, but rather children, whose age and subsequent reactions based on age, leads to their tragedy, as much as the 'ancient grudge.'
In a word, suspense! We already know in the first 12-15 lines what is going to happen in the entire play, including that the two will die. What we don't know is HOW it happens. The details make everything much more exciting and wonderful and tragic. Shakespeare does not give us the hero or heroine in the first scene in order to build up suspense for the rest of the story. It also serves as an introduction to the background of the feud between the two house--Montague and Capulet. Without the first scene where even the servants of these two houses are fighting, the audience would not have any idea as to how deep and how bitter the feud between the houses is. So, when Romeo and Juliet DO fall in love, there is even more suspense and curiosity about how they will ever possibly make their love work. We become so in tune with these two and hope against hope that the Prologue is incorrect...maybe, just maybe, they will be able to run away and live happily ever after. Then, when the prophecy of the Prologue plays out in full, we are even more saddened for the two young lives wasted over a silly argument that began so long ago that no one really even remembers the cause.
So, suspense! Drama! Shakespeare has an amazing way of making his audience sit on the edge of its seat just itching to know what happens next.