Romeo wants to go precisely because Rosaline (his latest love) is going to be there, but Benvolio wants to go so that both of them can indulge in other beauties. In regards to Benvolio, he spends the entire scene trying to convince Romeo that there are other fish in the sea. "Compare her face with some that I shall show, / And I will make thee think thy swan a crow." What is interesting is that it is impossible for the reader to tell whether Benvolio is really just wanting view the beauties for his own benefit or for Romeo's benefit. That's the beauty of theater: the scene could be played either way. In regards to Romeo, he believes none of it. "I'll go along, so such site to be shown, / But to rejoice in splendor of mine own." In other words, he has eyes only for Rosaline. Hmmmm, . . . just wait, Romeo. Just wait. Ha!
The good-natured Benvolio is concerned about his friend Romeo and the despondency into which he has fallen upon learning that Rosaline is going to a nunnery. So, having heard of the celebration to be held as a masque at the home of the wealthy Capulets when the illiterate servant bids him read the invitation Benvolio suggests to Romeo that he go along for some diversion and to make comparison of Rosaline with other beauties and he will see that his "swan [is] a crow"; in other words when he sees Rosaline next to other beauties, Romeo will realize that she is not so beautiful, after all. But, Romeo disagrees,
One fairer than my love! The all-seeing sun
Ne'er saw her match since first the world begun [sic] (1.2.94-95)
Benvolio does not agree; he says that Rosaline has only seemed beautiful to Romeo because there is no one against whom to compare her, but at the party she will not seem so. Reluctantly, then, Romeo agrees to go, not because there is anyone to compare to Rosaline, but simply in order to see her beauty one more time:
I'll go along no such sight to be shown,
But to rejoice in splendor of mine own. (1.2.102-103)
In Act I Scene 2, Benvolio and Romeo come across Capulet’s servant, who is charged with inviting the guests to Capulet’s ball; however, he cannot read the guest list, and must ask Romeo’s aid in the matter. In this way the two Montagues learn of the ball and of the many women who will be there – “Signior Martino and his wife and daughters,” some “beautious sisters” and “lovely nieces” – and of course Rosaline, Romeo’s lost love, destined for the nunnery rather than his arms. After Romeo has read the list, the servant invites him and his companion, on the condition they “be not of the house of Montagues.”
When Romeo and Benvolio first come onto the scene, Benvolio is trying to comfort Romeo after losing Rosaline. He argues that all his cousin need do is find another lover – "Take thou some new infection to they eye,/And the rank poison of the old will die." And yet Romeo will have none of it, preferring to be a prisoner of his misery. So, when they are invited to Capulet’s ball, the event falls perfectly into Benvolio’s attempts to distract Romeo. He states that at the party, Rosaline will be surrounded by “all the admired beauties of Verona,” and when Romeo sees her compared to such beauty, he will forget her instantly, and see how ignorant he had been for loving her.
Benvolio wants to go to the party to end Romeo’s moping heartache – he asserts proudly that when Rosaline is weighed against the other many women at the ball, “she shall scant show well that now shows best.” Romeo, the hopeless romantic, will not be swayed by this argument, but avows that he will “go along…to rejoice in splendor of mine own.” He will go for his own reasons – likely to see Rosaline again. So really, it is Benvolio who desires to go to the party, and who persuades Romeo to tag along in hopes that the feast will raise his spirits.
Romeo agrees to go to the party because he reads that Rosaline will be there. In Act I, Scene 1, the reader learns that Romeo is in love with Rosaline, unfortunately Rosaline does not love him in return. This makes Romeo sick with woe. In other words, he is love sick. No amount of consoling from Benvolio can diminish Romeo's despair. At one point Benvolio tells Romeo to forget Rosaline because there are plenty of other beautiful women in the world.
After reading the invitation and learning that Rosaline will be at the party, Romeo's interest is peaked. He wants to go, so that he can watch Rosaline. Benvolio encourages Romeo to go to the party, so that he can show Romeo a whole bunch of other beautiful women that will make Rosaline look plain in comparison.
"Go thither, and with unattainted eye Compare her face with some that I shall show, And I will make thee think thy swan a crow."