Romeo's character has been interpreted in a number of ways by critics, but most would agree that he is portrayed, certainly early in the play, as particularly prone to falling in love. He is in love with Rosaline at the opening of the play, and distraught because she will not return his love. Yet within moments after meeting Juliet, he has forgotten all about his former love interest and is completely infatuated with his new love. All of Romeo's friends, especially Mercutio and Friar Lawrence, chide him for this characteristic, which strikes them as fickle.
Related to this trait is Romeo's impulsiveness. After only a very brief encounter with Juliet, he is prepared to marry her. It is also true, of course, that she wants to marry him as well, so this characteristic is more a comment on the impulsiveness of youth rather than anything inherent about Romeo's character. Still, on multiple instances throughout the play, he takes actions without considering the consequences, such as when he kills Tybalt.
Finally, some readers might conclude that Romeo is still rather childish and somewhat self-absorbed. For example, when he learns that he is to be banished rather than executed for killing Tybalt, he throws himself to the ground weeping, and is rebuked by the Friar for doing so. He does not recognize his good fortune. However, it could be persuasively argued that this act only demonstrates the depth of his love for Juliet.