Answer:
When we first meet Romeo, he is despairing over the unrequited love of Rosaline. Since we see his decision to attend the Capulet party from his point of view, we know he is going in hopes of seeing Rosaline. By introducing the audience to Romeo before he meets Juliet, the play lets us see who he is as an individual, and how he is changed by love. Romeo initially seems more in love with the idea of love than Rosaline herself. He suggests that love is madness, and fleeting. Romeo suffers because he thought he was in love, but his girl decided she wanted to be chaste. To him, love is fickle and changeable. In a way, Romeo's complaints foreshadow what is about to happen.