What factor is most responsible for difficulties facing Romeo and Juliet?
We learn in the Prologue to Act One that Romeo and Juliet are "star-cross'd lovers," and that their love is doomed to end in death shortly after it blossoms. So it would seem that fate is really the factor most responsible for all of their difficulties. We see this throughout the play, and particularly once Romeo has been banished to Mantua, when the message Friar Laurence sent him alerting him of the plot to reunite him and Juliet failed to arrive (because of plague in the town.) So the lovers are struggling against fate. However, the other factor is the longstanding feud between their respective families. But for that, their love would not have been forbidden. Because Romeo is a Montague, he is the mortal enemy of Juliet's family, and this, of course, has tragic consequences. It forces them to marry in secret and it results in his killing of Tybalt. Indeed, there is a sense in which the two young lovers were sacrificed to bring peace to their families. So fate and the old family feud between their respective families are the sources of Romeo and Juliet's difficulties.