Why does Romeo want th sun to kill the envious moon?

Romeo says this during the balcony scene and right after he meets Juliet during the party. The moon is symbolizing Romeo's love for Rosaline and the sun is symbolizing Romeo's love for Juliet. So, Romeo wants his love for Juliet to rise up and be greater than and overcome his love for Rosaline. Why can't this child stick to one girl? Quite a player! :) ;)