It is presumed that the Montagues and the Capulets hated one another because both families wanted to be the most powerful in Verona. The famous play “Romeo and Juliet" by William Shakespeare never fully explains the reason behind the feud, though it is assumed to have lasted for many years.
One possibility for the family feud is the betrayal of one family by the other. Another is that rumors were spread to other families in the city. Unfortunately, the feud between the Montagues and Capulets does not end until Romeo and Juliet kill themselves because their families couldn't accept their love for one another.