At the start of Romeo & Juliet, we are introduced first to servants of the two feuding houses as they walk down the street of Verona, where the scene is set. While we don't yet know what exactly has caused the feud between the Montagues and the Capulets, we do know that both families are willing to duel to the death to uphold their honor and to defeat the other. The Prince, upon arriving at the scene, declares "If ever you disturb our peace again Your lives shall pay the forfeit of the peace- (1.1.99-100).
We are first introduced to Mercutio when the Montagues are on the way to the ball at the Capulet's in scene 4; he and Romeo enter into a heated discussion on the definition of love. Mercutio's goal in the scene seems to be to mock Romeo for his love of Rosaline, which Mercutio views as not being love at all. He views Romeo's love of Rosaline as being blindly self-indulgent because he gives no thought as to the desires of Rosaline, and is too busy thinking of himself. Mercutio seems to view love as simply an excuse for sex, and doesn't believe that romantic love truly exists.
At first, Mercutio tries to talk Romeo out of being melancholy and depressed over his love for Rosaline, but Romeo complains that he has been too sorely wounded by her lack of interest in him to even consider having fun. Mercutio suggests that Romeo .
"Borrow Cupid's wings and soar with them above a common bound- (1.4.17-18), where he is telling Romeo that love can give him the power to make a tremendous leap in the dance of love if Romeo will only give it a try.
Romeo, however, responds with that he cannot because: "I am too sore enpierced with his shaft To soar with his light feathers; and so bound I cannot bound a pitch above dull woe. Under love's heavy burden do I sink- (1.4.19-22). Romeo is essentially saying that he can't make huge leaps because he has been so damaged by Cupid's arrows that he has not the strength to borrow Cupid's wings.