Rosaline rejected his affections since the feelings were not mutual and she chose to live a life of chastity. He whines to his cousin Benvolio, who says: "Be ruled by me. Forget to think of her." (Romeo & Juliet. 1.1.216.) to which Romeo replies with: "O, teach me how I should forget to think!" (Romeo & Juliet. 1.1.217.). Benvolio follows up with more advice: "By giving liberty unto thine eyes. Examine other beauties." (Romeo & Juliet. 1.1.218-219.).
Next, Romeo and Benvolio hear about the Capulet party from the illiterate servant Peter. Peter asks Romeo to read the letter for the Capulet party. Benvolio suggests they crash the party. Romeo agrees only because he desperately wants to see Rosaline. "I’ll go along, no such sight to be shown, But to rejoice in splendor of mine own." (Romeo & Juliet. 1.2.102-103.).
Then, when Romeo first sees Juliet, he starts tediously discussing her beauty. Tybalt overhears Romeo talking about his young cousin Juliet and gets angry, even going so far as to call Romeo a villain, foreshadowing the fight between Tybalt and Romeo that kills Mercutio and Tybalt. "Or the "villain" may consist in a certain chemical interplay of the passions of two or more characters."