Benvolio: Tybalt, the kinsman of old Capulet,
Hath sent a letter to his father’s house.

Mercutio: A challenge, on my life.

10
Benvolio: Romeo will answer it.

Mercutio: Any man that can write may answer a letter.

Benvolio: Nay, he will answer the letter’s master, how he dares, being dared.

Mercutio: Alas! poor Romeo, he is already dead; stabbed with a white wench’s black eye; shot through the ear with a love-song; the very pin of his heart cleft with the blind bow-boy’s butt-shaft; and is he a man to encounter Tybalt?

Why does Shakespeare include Mercutio’s teasing words?

to explain the history of the feuding families
to criticize Romeo’s neglect of his duties
to condemn the established ritual of sword fighting
to contrast the seriousness of Tybalt’s challenge

William Shakespeare's "Romeo and Juliet" is a play about two lovers from conflicting families, the house of Capulet and the Montague. And the fatal repercussions of this relationship will lead to the two families to mend their enmity.

In Act 2 Scene 4 of the play, Mercutio and Benvolio were outside in serch of Romeo for he hadn't returned home the previous night after the party. Benvolio mentions that Tybalt, the relative of capulet had challenged Romeo to a duel. It was then that Mercutio said the lines where he said that Romeo might not be man enough to fight. For he had been blinded by love. Mercutio is amused at the thought of Rome fighting Tybalt in a fight, as they are complete opposites. Tybalt is a good swordsman while Romeo is a romantic lover.