The scene that you are referring to occurs very early in the play, in Act One, Scene One to be specific. Romeo, in multiple instances and many round about word plays, tells Benvolio that he is in a love to beat all loves. The scene is basically a cliche of every teen romance movie ever made. Rosaline is super beautiful, Romeo is awestruck by her beauty, she is perfect, she is the only one for Romeo, it hurts to think about being away from her, etc.
Romeo is one love struck depressed guy. Benvolio greets Romeo with a "good morning" and Romeo's response is "Is the day so young?" Basically Romeo is so hung up on Rosaline that he is shocked that so little time has passed during his depression.
At this point, you may wonder why Romeo is so depressed if he is so in love. Benvolio asks the same thing, and that's when the reader finds the source of Romeo's lovesick depression. Rosaline keeps rejecting every advance of Romeo's, and she is not just playing hard to get. Rosaline has taken a vow of chastity. Poor poor Romeo. Benvolio's response is great at this point. He tells Romeo to get over it by "examin[ing] other beauties." In other words, "there are plenty of fish in the sea, go find another beauty."