They have great control over their lives. They are constantly making decisions, often rash ones, that lead to their untimely deaths. Their inability to be patient and love moderately as Friar Lawrence warned Romeo, was responsible for their deaths. Their unwillingness to be straightforward with their love and instead put people like Friar Lawrence in difficult situations played a large part in their death. Finally, of course, their impulsive behavior and fascination with death sealed their fate. Whenever the possibility of living without the other arose, their first thought was always suicide. Even when they were talked down from such drastic measures, it always remained firmly in their heads. That's why it's not so surprising that they each chose to take their own lives at the end. Notice the key word being "chose" meaning they had a decision and fate had little to nothing to do with it. Sure, they ran into some bad luck and challenges of being together, but I haven't met a married couple who hasn't faced these difficulties to some extent.