ROM: Give me a torch. I am not for this ambling.
Being but heavy, I will bear the light. (I.iv.12=3)
Romeo is, in a word, depressed. He has been sorrowing all of this Act, so far, over the fair Rosaline, with which he has had a failed (or perhaps never-started) love affair. All of his friends are, in the manner of young men, ready to crash the party of their rivals the Capulets. This is a dangerous proposition, but also an act of bravura which would win them admiration among their friends. But Romeo is dragging his feet, and barely agrees to go along. He calls himself "heavy", meaning sad, and continues the metaphor later by saying that his soul is made of lead, and therefore is too heavy to go dancing at the Capulet's house. When Mercutio mentions that Romeo is of an amorous bent, and, presumably, there will be young ladies at the Capulet's, Romeo retorts that he has been too badly wounded by Cupid's arrow to carry on as a lover. Mercutio and Benvolio try very hard to cheer Romeo up, but he is having none of it. In addition, Romeo has a premonition of something happening on this day:
ROM: I fear, too early; for my mind misgives
Some consequence, yet hanging in the stars,
Shall bitterly begin his fearful date(115)
With this night's revels and expire the term
Of a despised life, clos'd 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! (I.iv.113-20)
In the manner of many Shakespearean tragedies, the end of the play is foretold. Romeo is not just sad about his love for Rosaline, but is actually feeling that something will happen that night which will end in his early death. We, the readers, will find out that this premonition comes true.