Romeo's tragic flaw that leads to the tragic denouement is actually his youth. It is important to remember that both Romeo and Juliet are very young in this play. Juliet is only 12, two years before turning 14, while Romeo is a bit older, probably in his late teens or early twenties. However, the age of late teens or early twenties is still a very naive and even stubborn age.It is Romeo's youth that leads to his intense emotionalism and his stubbornness, which are both attributes that lead to his tragic denouement.
We see him react with intense emotionalism with respect to his response at being rejected by Rosaline and also with respect to falling in love so deeply and so suddenly with Juliet. We learn how profoundly Romeo is feeling his rejection by Rosaline, when we learn that Romeo has been seen crying at dawn each morning under a tree on a certain side of town, possibly the side of town in which Rosaline lives. We learn this from Romeo's father who says,"Many a morning hath he[Romeo] been seen, / With tears augmenting the fresh morning's dew" (I.i.127-128).
Romeo's youth also leads to his stubbornness. We see him being stubborn in his refusal to listen to any advice. For example, in the opening scene, we see him refuse Benvolio's advice to "be ruled by [him]: forget to think of her [Rosaline]" (227). We also see him refuse to listen to Friar Laurence's advice, who makes it very clear he thinks that Romeo does not yet truly know what love is and that he is being hasty in his decision to marry Juliet. We see this when Friar Laurence cautions, "Wisely, and slow. They stumble that run fast" (II.iii.97).
Romeo's intense emotionalism and stubbornness, which are a result of his youth, lead to his tragic denouement. It is his intense emotionalism that drives him to revenge himself on Tybalt for Mercutio's death when he knows that the law would have killed Tybalt otherwise. Killing Tybalt leads to his banishment, which leads to Juliet's faked death and ultimately his own death. His stubbornness also leads to his tragic denouement because his stubbornness leads him to marry Juliet hastily, which leads to problems for both of them, ultimately causing their deaths.