Why does Romeo feel uneasy about going to the party?

Romeo gets all prophetic by the end of this scene. Mercutio has just finished his way too emotional "Queen Mab" speech and Romeo feels a strange foreboding in the air. This is Shakespeare's not so subtle foreshadowing,

I fear, too early: for my mind misgives

Some consequence yet hanging in the stars

Shall bitterly begin his fearful date

With this night's revels and expire the term

Of a despised life closed in my breast

By some vile forfeit of untimely death.

But He, that hath the steerage of my course,

Direct my sail! On, lusty gentlemen.