Why does Mercutio curse both the Capulets and Montagues when he dies?

Mercutio curses both the Capulets and Montagues as he dies in Act 3, Scene 1. His curse "a plague o' both your houses!" shows that he blames the two families for killing him.
Mercutio believes that he had to fight Tybalt to protect the Montague reputation. By extension, Mercutio also blames Romeo for his fatal sword wound. In Mercutio's mind, Romeo's intervention caused him to sustain the bloody injury.
Additionally, Mercutio is angry that Tybalt managed to get the best of him. It's obvious from Mercutio's comments (to Romeo) that he doesn't think very much of Tybalt's sword-fighting skills. In fact, Mercutio thinks that Tybalt fights like he learned all his moves from a manual of sorts. Yet, to Mercutio's humiliation, Tybalt managed to get away without so much as an injury.
So, the curse serves a few purposes. First, it is one way for Mercutio to voice his distress; after all, he is dying. Second, he wants Romeo to know that he blames the feuding families for his fate. Third, Mercutio wants Romeo to know that he includes him in the insult; after all, it was Romeo's intervention that caused him to sustain his fatal wound.