Tensions

"Two households, both alike in dignity,
      In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny
      Where civil blood makes civil hands unclean." (Prologue; 1-4)

The two households that this Prologue speaks of are the Montagues and the Capulets.  Being "both alike in dignity" means that both households have the same the social status.  The play does not specify why exactly these households have this grudge.  We can infer that the households themselves have forgotten the reason for their hate, they just do.