The obvious answer to this question is that the feud between the Montagues and the Capulets causes the death of Romeo and Juliet. Somewhat less obvious is that it causes their love, or at least the type of love for which the play is famous.
The Montagues and the Capulets are two of the leading families in Verona. If they were not feuding, they would constantly be mixing in society. Romeo would certainly have encountered Juliet as a very young child. It is quite likely, in fact, that their families would have arranged a marriage between them, since Romeo would have been a highly eligible husband for Juliet and such a match would have united their families and fortunes.
It is quite possible that Romeo and Juliet would still have loved each other, but it would have been a very different type of love: the affection of childhood sweethearts who had already known each other for a decade or so before the play begins. Or they might have come to be more like brother and sister after so many years in...
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