The animosity between the Capulets and the Montagues is only part of what contributes to Romeo and Juliet's becoming "star-crossed" lovers, doomed from the beginning. Even the light/dark imagery employed by Shakespeare in Romeo and Juliet conveys this idea. For, in the daytime, the lovers are not safe together; only in the darkness can they love. Thus, their secrecy, forced by the conditions of the feud, leads to the complications that turn to tragedy.
Since Romeo cannot ask Lord Capulet for permission to marry Juliet and secretly weds her instead, Paris unknowingly asks for the hand of the young maiden. The complication of this marriage proposal to Juliet's life cannot be resolved because Juliet dare not reveal to her mother that she has married one of their enemies. Thus, she panics and flees to Friar Laurence, who unwittingly sets in motion the tragic events at her tomb.
When Tybalt, who has become enraged by Romeo's presence at the party for Juliet, walks the streets of Verona, his enmity causes him to engage in conflict with the loyal friend of Romeo, Mercutio, who is fatally wounded during Romeo's well-meaning intervention. Without this enmity between the two families, Romeo and Mercutio would not conflict with Tybalt, and neither would be placed in danger. Romeo's resulting banishment has resulted from this conflict, a banishment that has caused him to remain ignorant of what transpires with Juliet's supposed death.
Because of the feud and its ramifications, Romeo is unable to defy the stars as he avows, in several circumstances. However, it is yet his impulsive behavior and rash judgment that leads to his tragic end, not the feud itself. Likewise, Juliet places herself in precarious situations because of the secrecy she becomes involved in as a result of marrying a Montage; but, again, she, too, dies from impetuosity, not hatred as she remains in the tomb instead of accompanying Friar Laurence out of it. There, she rashly decides to join Romeo in death.
First, it makes Tybalt mad at Romeo. So when he tries to fight Romeo and Romeo ends up killing him. That makes it so that Romeo gets exiled and Juliet is sad.
Second, Juliet cannot tell her parents that she is in love with Romeo and that they have actually already gotten married. That means that they still want her to marry Paris and she has to take the potion that Friar Lawrence makes so that she can play dead and not have to marry him.
Because she takes the potion, Romeo thinks she's dead and kills himself. She wakes up, sees him dead and kills herself.
there is a long lasting fued between the Montagues and the Capulets. So when Romeo kills Tybalt, Prince Escalus bans Romeo from the city, leaving Juliet behind.
There are many occasions in the play where the feud stems a chained reaction of tragedies and directly influences the the final tragic outcome. However, there are a few points or occurences of sheer bad luck, like the delay of the letter to Romeo, which results in both the lover's tragic deaths.