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When we first meet Romeo, he's mooning over a girl named Rosaline. Romeo has a crush on her and she continually rejects him. Romeo's cousin Benvolio encourages Romeo to go to the Capulet ball to distract Romeo from his melancholy. Rosaline will certainly be at the ball, but so will many other young ladies. Benvolio hopes Romeo will meet someone new and forget Rosaline.
"Take thou some new infection to thy eye,
And the rank poison of the old will die."
Benvolio knows that Romeo is heartbroken because of his unrequited love for Roseline. Benvolio argues that Romeo should go to the Capulet feast and look at other women. He believes, correctly, that all Romeo needs to get over his depression over Rosaline is to look at other beautiful women, and compare them to Rosaline. Benvolio argues that Romeo's love for Rosaline will fade when he compares the beauty of other women to Rosaline's beauty. Benvolio understands that Romeo's love for Rosaline is shallow and can be categorized more as lust than true love. He is also aware that his cousin Romeo is fickle and emotional. Benvolio is right because, as soon as Romeo sees Juliet, all thoughts of Rosaline disappear. It's the old "there's more fish in the sea" argument.