Why shouldn't Romeo and Juliet fall in love in Romeo and Juliet?

One thing: They DID get married!

But anyways, the reason is because the Montagues and Capulets were basically "at war" with each other. There were riots in town squares, tension boiling, and other acts of violence.

The Friar WANTED them to get married because he thought that if they did, the fighting would stop between the two families.