Romeo want to break Benvolio's shin because he wanted him to forget about his love.
Step-by-step explanation:
Romeo is a lovelorn soul in the play and he has seen himself enamoured by a woman that he must have.
Benvolio as his friend tries, and fails miserably to be the voice of reason to him in that he tells him to be realistic and not fall for love that is unattainable and undesirable.
However, Romeo understands the demeaning tone in the voice of Benvolio and thus wants to break his shin for the insolence that Romeo will not be able to fulfill his love and will fail.