why are the Montagues and Capulets fighting in the first place?

Both families are involved in an "ancient feud" or grudge. We are not really given a reason why other than they have been fighting for a very long time.

What really causes the fight is the fact that the Montagues and Capulets hate each other. They hate each other so much that even their servants hate one another. It is these servants who are the immediate cause of the fight. Sampson and Gregory (Capulet servants) are walking along when some Montague servants come out.