As I was saying. Shakespeare borrowed more than one plot from other sources, which takes nothing away from his greatness, of course. However, the most common foreign setting for his plays was Italy, giving rise to all sorts of speculation about his travel and his knowledge of that fair country. Some have gone so far as to maintain that Shakespeare did not write the plays, but this is too stupid for me to go into, so I'll just skip it.
First of all, you should know that the Italians call the fabled pair Giulietta e Romeo (jool-YET-tah eh roh-MEH-oh), always in that order. The story of the two was set forth in a short prose work by an unhappy nobleman from Vicenza, Luigi da Porto (1485-1529). He composed it while at the family's villa in Montorso Vicentino, stating unromantically that its purpose was to warn people off from the dangers of love. He writes that he is merely setting down a well-known tale, as told to him by a man from Verona. The setting is also in Verona, as are the characters. Montorso is near Vicenza, off the road that goes to Verona.
I just finished reading Da Porto's original, and it is strikingly similar to the Shakespeare play. However, Will did embellish and lengthen it (not to mention improve it by a factor of a million). To further complicate matters, an even older version of the story was apparently penned earlier by a Masuccio Salernitano, a Southerner, as his name would indicate. His lovers had completely different names. Not complicated enough? The surnames of Romeo and Juliet, respectively, are given by Luigi da Porto as Montecchi (Montague) and Cappelletti (Capulet). However, these names were already associated centuries earlier with warring factions, as cited by Dante in the Purgatorio. The version by Masuccio is quite dissimilar from the Bard's play, and does not contain the surnames of Montecchi and Cappelletti.
Add to this the fact that from Da Porto's villa one could see the castle of Montecchio (similar to the name Montecchi) (see above), now known locally as Romeo's castle (I've been there), and this leads me to believe that we should credit the obscure man from Vicenza as the "father" of the star-crossed lovers. It seems to me that a combination of his life experience, his setting near the castle of that name, and a reading of the names of Montecchi and Cappelletti as feuding factions in Dante led him to concoct the story in the form later adopted by Shakespeare through English translations. But I make no claim to scholarly rigor.