Fate is defined in many ways.  There are two definitions that work best to describe the situation of Romeo and Juliet.  The first definition states that fate is "that which is inevitably predetermined; destiny".  The second definition says that fate is "something that unavoidably befalls a person; fortune."  Using these two definitions, it is easy to see why it is considered fate that Romeo and Juliet were born into the families that they were born into.  Using the first definition, there is/was no way for Romeo or Juliet to know that one day they would meet and fall in love.  It was simply a case of being in the wrong place at the wrong time or the right place at the right time, dependent upon whether one thinks their meeting was a positive or negative event.  Fate can not be changed, nor can it be predicted.  If one could have predicted that this would have happened to them, maybe the events in the story would have turned out differently.  Since fate is unavoidable, this is why it was fate that the two were born into feuding families, no one could have predicted it or changed it.