Ask Billy Shakespeare. Pretty sure HE caused the fight, and the death of a perfectly lovely couple. LOL

Two Words: Jamie Kennedy

In those days? You're funny. Even today neighbourhood disputes can go on forever and become insanely ridiculous, atleast in suburbian or rural areas where you still know your neighbours.

As far as the film goes, I assumed the feud was about territory/big business. They both had a huge corporate headquarters right next to each other, and some of the shots in the beginning show various properties/companies owned by each.

Who knows. In those days, things that would seem so stupid and trifling by our standards would've caused generations long feuds. From the sounds of things, it was probably something stupid and trifling by THAT era's standards as well.

imagine they were the coke vs pepsi of their time

Someone stole someone else's goat and it escalates over generations.
People don't like to be meddled with.

~Memories made in the coldest winter~

Of course it falls apart when you realize Macs are Pc's.

Montague is a Mac. Capulet is a PC.

No reason is given for the feud. They have apparently been at odds forever. They would have made Juliet get an annulment and marry Paris. Or they might have realized that Romeo and Juliet were an acceptable match and made peace overtures to each other.
"Joey, have you ever been in a Turkish prison?"

Comparably to the Hatfields and McCoys, no one actually knows and think it not being revealed to us, further drives the point home of it being a dated, insignificant conflict.

'I think Shakespeare intentionally left out the cause of the feud, to make the conflict in the play seem even more empty and absurd. '
Good point.

I think Shakespeare intentionally left out the cause of the feud, to make the conflict in the play seem even more empty and absurd.

Can't be it. The play is set in Italy. Catholics ruled there and plus the grudge was ancient at the time the play was written, the Protestants were only near 100 years old.
Tangled up since 12/31/10

In the prologue, Shakespeare refers to the feud as an 'ancient grudge' so it has presumably been passed down through the generations. As is so often the case, the original cause may well have been forgotten over possibly hundreds of years,

There is an author named Donald Miller who supposed that they were fighting about the old 'Catholic vs. Protestant' thing that was so enduring in England at that time. It kind of is a 'Face-saver' that he doesn't really point it out, though, isn't it?

It is never stated why the two families are arguing. In all possibility, it is a feud that has lasted generations, so not even the modern day families know why they are fighting, they just know that the other family is their enemy.

William Shakespeare never mentioned why the two houses were fighting so it's anyone's guess. Chances are it was something going back a long time since it seemed to be old news to the rest of the town.
What would've happened after the marriage was revealed is also anyone's guess. Personally, I think the families would've made up or at least come to a tolerance of each other since the the two fathers immediately reconciled after seeing the devastation their feuding had cause.