What is the basis of the enmity between the Capulets and the Montagues?
The enmity between the Capulets and the Montagues is the basis for the central tragedy of the play -- the deaths of Romeo and Juliet. However, the basis of this enmity is really not very important. We don't need to know *why* the families don't like each other -- all that is important is that they don't.
This is a good thing, because we are never told why the two families dislike each other. All we really know is what is in
The Prologue -- they have disliked each other for a long time and now their feud is about to be ended by the death of a child from each house.
We are never told the basis for the hatred between the Montagues and the Capulets, only that it was an ancient grudge that has broken out anew, presumably because of an event between the servants of the two houses. The fact that the grudge between the two houses has no cause makes it all the more pointless, and more devestating in its destruction.