Romeo and Juliet were young teens who were not ready for the committment of marriage. Their actions were impulsive and compounded the tragedy. First, Romeo was in love with Rosaline and pined over her even when she never gave him the time of day and didn't know he existed. Then all of a sudden he sees Juliet at the feast, and Rosaline no longer exists. Next, Romeo and Juliet make their vows to each other, on the balcony, when they have only muttered a few words at the feast and they are already making wedding plans. Soon they are married and before the marriage even gets started, Romeo is banished, Juliet fakes her death, Romeo thinks she is dead, she drinks the poison, and Juliet stabs herself. The two were impulsive in their actions, but determined to be together. They didn't care what their families did or said, they would be together, whether in life or death. They knew what the outcomes could be, but it didn't stop them. But even if the couple took their time getting to know each other, or courting as they say, the odds were against them from the start because of the Feud. Did their impulsive actions cause the tragedy, partly, but their family history would never have allowed them a future together.
Romeo and Juliet were children (at least minors) by today's standards. Many people would say they were too young to know what love is, that they were just infatuated with each other. They never spent enough time together to find out whether either had any annoying habits. So in that sense you can say that their impulsive actions caused their tragic ends. Come on, fake your death to make your parents let you have your way? Foolish choice. But they believed they had no other choice. If their parents had acted differently, maybe Romeo and Juliet could have gotten to know each other and not have rushed into a serious relationship.