Romeo went to the Capulet ball in order to see his crush Rosaline.
Act I, scene ii, lines 102-103
Romeo's friend Benvolio claims that the only reason Romeo thinks Rosaline is so great is that Romeo saw her when she was all alone, so there was no one to compare her to.
So has been egging on Romeo to go to the ball so he can compare Rosaline to other women. Benvolio thinks that comparing her will let Romeo realize that she's not as beautiful as Romeo thinks she is.
Romeo agrees to go, but he specifies that he isn't going in order to comply with Benvolio, but just for the chance to see Rosaline herself,
The passage is from Act I, scene ii, lines 102-103
Tut, you saw her fair, none else being by,
Herself poised with herself in either eye.
But in that crystal scales let there be weighed
Your lady’s love against some other maid
That I will show you shining at the feast,
And she shall scant show well that now shows best.
I’ll go along, no such sight to be shown,
But to rejoice in splendor of mine own .
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