Romeo does have flaws. He is in love with being in love and lives too much in the moment. Whatever emotion he is experiencing at a particular moment becomes the be-all and end-all of his life. Like many adolescents, he can't seem to think even a few days ahead of the all-important now.
This flaw first emerges, rather comically, in his love for Rosaline. When she won't return his love, he moons around, worrying his father. When his friends suggest a visit to the Capulet ball, he agrees unwillingly to go. He says there is no possible way he could ever think of any other girl in the world other than Rosaline. However, he has hardly been at the Capulet's two minutes when he falls helplessly, hopelessly in love with Juliet. Rosaline is utterly forgotten.
This impulsive trend continues. Not only does Romeo now consider Juliet every woman in the world—along with better than the sun and all the stars in the sky put together—he wants to marry her the next day and won't rest until this happens. A more sensible person might say, "let's slow down and get to know each other better," but that is not Romeo's way.
This leads to Romeo's destruction when he finds Juliet seemingly dead. Rather than take time to reflect and try to think through a sensible plan of action, he acts out of the emotion of the moment and kills himself. Ironically, Juliet isn't even dead.
Romeo takes to an extreme the concept of love at first sight and acting immediately on one's desires. The friar warns him to slow down and not let his passion burn too bright, but Romeo is not structured to listen to that kind of advice.