Benvolio's Reason:
Benvolio is concerned about Romeo's recent melancholy and believes attending the Capulets' masquerade ball could be a distraction and possibly improve his mood. He argues that Romeo should go to observe the beauty and charm of other women, hoping it might help him forget his unrequited love for Rosaline.
Romeo's Agreement:
At first, Romeo is reluctant to attend the Capulet's party. He sees it as a foolish and pointless endeavor. However, Benvolio's persuasion eventually sways him. He eventually agrees to go to the party, partly out of curiosity and partly to please his friend.
It's important to note that Romeo and Benvolio's conversation about attending the Capulets' party occurs in Act 1, Scene 2, setting the stage for the fateful meeting between Romeo and Juliet.