How did shakespeare show the inevitability of Romeo and Juliets love ending in tragedy?

“Tragedy, then, is an imitation of an action that is serious, complete, and of a certain magnitude”, translation by S.H Butcher. Aristotle tells us that drama is shown by drama, not told in narrative and is more philosophical than history because history tells us what has happened rather than tragedy dramatizing what may happen.
Events that have happened may be subject to coincidence or accidents, and may not have a clear cut cause and effect chain that reveals what may happen at any time and place because it is the way that the world operates, meaning they have little relevance to other people. Because of this, tragedy not only arouses pity but also fear, because the audience can envision themselves inside this cause and effect chain.
Katharsis is an Aristotelian term. It has caused a lot of debate. The word itself means “purging,” and Aristotle seems to be using a medical metaphor—tragedy arouses the emotions of pity and fear in order to purge away their excess,...