"Wherefore" means "why." That's what the word means. In the context of the play Juliet is lamenting falling for a guy whose family is in a feud with hers.
It means where. Fore is just kind of extra fluff in this case. It might imply in the area before the asker, but it usually isn't interpreted too strictly.
@Xuan12 @NorthernRoses "she asked why he just [i]had[/i] to be Romeo, of all people" or why the person she loves just [i]had[/i] to be a Montague, of [i]all[/i] families
'why'. juliet is beside herself with sadness, because the person she loves is also a member of a family that her family hates - - the family feud is keeping them apart and she is torn.