Because their families are involved in a deadly feud, Romeo and Juliet know they have to keep their love secret. Juliet is well aware that if her family discovers Romeo is in love with her and wants to marry her, her male relatives will kill him. Romeo knows his family will not allow him to marry a Capulet. As a result, the two marry secretly.
Although Romeo would, at this point, love the feud to end, the reality is that Tybalt, Juliet's cousin, is spoiling for a fight. Romeo's flirtation with Juliet at the Capulet's masked ball is Tybalt's latest excuse for provoking violence. Romeo ends up killing him after Tybalt kills Mercutio. Romeo is banished for the act, which throws Juliet into a state of deep grief. Juliet's father misinterprets Juliet's grief as due to her beloved cousin Tybalt's death. His answer is a speedy marriage between Juliet and Paris, which he believes will get her mind off her problems.
Instead, it creates a new crisis, leading to Juliet taking a potion that feigns death. Romeo finds her and thinks she is really dead. He kills himself. Awakening and finding Romeo dead, Juliet then kills herself.
In sum, the feud leads to the lovers' deaths in numerous ways: It causes them to secretly get married. It causes Romeo to accidentally make it possible for Tybalt to kill Mercutio, leading to Romeo killing Tybalt and being banished from Verona. It causes Lord Capulet to order an early marriage between Paris and Juliet, interpreting Juliet's distress as due to Tybalt's death. It then causes Juliet to pretend to death rather than admit she is married to Romeo, leading both lovers to commit suicide. Without the feud, the two lovers, both from wealthy and noble homes and loved by their parents, who wanted them to be happy, could have married without a problem.