Why are Romeo's and Juliet's families against each other?

We don't really know exactly why the Montague and Capulet families are fighting with each other.  In the Prologue, the Chorus says that they are "Two households, both alike in dignity" (line 1).  The families are of equal status and honor (and wealth, too, we can assume), and they have between them "an ancient grudge" (line 3).  The feud is apparently so old, then, that no one ever even discusses how it originated; they just continue to brawl nonetheless.  The only thing the Capulets and Montagues seem to fight about in the play is family honor: whose master is better, whether or not Romeo, a Montague, should have been in attendance at the Capulet party and whether his presence dishonored the Capulets (according to Tybalt, it does).  None of the characters reveals the original cause of the fighting between the families, and so we never learn it.