In Act 1, Scene 2 of Romeo and Juliet, why does Romeo want to go to the Capulet's party and why does Benvolio want to go with him?

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.