In the prologue it says that the hatred between the two families is ancient. The Montague's and Capulet's hated each other, because their families have been at an ancient feud and they are just maintaining what their ancestors did. ... They hate each other because of the reason for the feud between them.
They hated each other so much that the servants even hated their servant
The reason for the conflict between the Montagues and the Capulets is never revealed in the play. The play's Prologue reveals that there is an "ancient grudge" between the two families. Additionally, Prince Escalus mentions their conflict and its effects on Verona, but again there is no reason given for it.
Hope this helps