Why does Mercutio say “a plague o’ both your houses”?

Explanation:

In Shakespeare’s play Romeo and Juliet, Mercutio utters the famous line, “A plague o’ both your houses.” This line is spoken immediately following his mortal wound in a sword fight between Romeo’s Montague family and the Capulet family, with whom Juliet is affiliated. The line may be interpreted in the context of the play’s larger themes of familial strife and violence. Mercutio’s words directly address the warring houses of the Montagues and Capulets, both of which he associates with his own demise.