In William Shakespeare's Romeo and Juliet, the conflict between the title characters is a result of the feud that exists between their two families, the Montagues and the Capulets, both of whom live in Verona, Italy. The third line of the play tells us that the quarrel was "From ancient grudge". The prologue also reveals that what happened between Romeo and Juliet was a "continuance of their parents' rage".
Later in Act I the Prince of Verona notes that on three occasions the city has been disrupted by "brawls" between the two families. After the Prince's exit, Montague again refers to the feud as an "ancient quarrel", but never specifies the reason for that quarrel.
This silence about the original cause of the quarrel between the Montagues and Capulets remains throughout the play. So, the bottom line is that Shakespeare never tells us why the quarrel started.