How does Romeo and Juliet's love affect the play?

Romeo and Juliet's love is crucial to the plot of the play.
First, their love reveals how arbitrary and unnatural the feud between their two families is. Romeo and Juliet have no reason to hate each other or to be separated, except for the long quarrel between their two houses. Nobody seems even to remember the reason for the fighting.
Second, because Romeo is in love with and has secretly married Juliet, he intervenes in the sword fight between Tybalt, Juliet's beloved cousin, and Mercutio, his own dear friend. This allows Tybalt to kill Mercutio, and, in grief and rage, Romeo kills Tybalt. Romeo's murder of Tybalt forces Romeo into exile.
Because the love between Romeo and Juliet has caused them to marry secretly, Juliet has to pretend to be dead to avoid marrying Paris. This leads to Romeo misconstruing her fake death as a real death and killing himself. His suicide, in turn, leads to Juliet's suicide.
Romeo and Juliet's love for each other causes both of them to die, but their deaths help heal the rift between their families.