Explain Romeo's motive for going to the Capulet party.

Romeo did not want to go to the party.  In Act 1, Scene 1, he is bemoaning the loss of his last love, Rosalind.  Benvolio is tired of seeing his friend mope.   Romeo asks how he can ever forget such a beauty.  Benvolio replies, "By giving liberty unto thine eyes: / Examine other beauties"  (1.1.225-26). 

Romeo still stubbornly wants to wallow in his misery, so Benvolio suggest that they attend the Capulet ball and prove to him that there are many other fish in the sea.   For added incentive, he suggests that seeing Romeo in attendance will make the fickle Rosalind jealous.   "I will show you at this shining at this feast," Benvolio promises  (1.2.100).  Romeo reluctantly agrees.  "I'll go along, no such sight to be shown, " he says, "But to rejoice in splendour of mine own (1.2.102-3).

Of course, Benvolio is quickly proven correct.  Romeo soon sees the lovely Juliet and falls madly in love.