Both Romeo and Juliet are impulsive. Romeo is the most impulsive. He is in "love" with Rosalind one minute, with Juliet the next. He agrees to marry within a short time of meeting Juliet. He is also secretive. He doesn't tell his parents or even his friends about his plans. If he had told Mercutio that he was married to Juliet, he would not have "shamed" Mercutio by refusing to fight Tybalt, and Mercutio would still have been alive. Had he told Tybalt directly instead of speaking in riddles, the fight may not have happened, and he may not have been banished. Juliet also moves quickly and keeps secrets from the people who should know.
They also place their trust in the wrong people. The Friar, though kind, makes some strange decisions for a person of his status. The Nurse, also, is too wrapped up in her own romantic version of the world to give Juliet sage advice.
They are too young, and they base their impressions on what they see on the surface. That is how they "fall in love" and that is also how they make the decisions that lead to their death. Romeo does try to get to the truth when he first learns the news about Juliet; however, he does not try hard enough. Both act on impulse without waiting to discover whether or not things are truly as they appear.