Romeo. Peace, peace, Mercutio, peace!
Thou talk'st of nothing.
Mercutio. True, I talk of dreams,
Which are the children of an idle brain,
Begot of nothing but vain fantasy,
Which is as thin of substance as the air
And more inconstant than the wind....
(Romeo and Juliet, 1.4.596 - 600)
Romeo and Juliet, those legendary lovers, live in a world of signs and tokens, where the influence of the stars shapes destiny and dreams, in some mysterious way, come true. Romeo doesn't want to go to the Capulet's party because of a dream, and his mystical feelings about the evening come true:
...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.
Yet, there's also a sense in the play, as expressed by Mercutio, that these dreams are "nothing," that there's an unreality, a fantasy that is at play. But what does that say about life, when the lines between dreaming and waking are blurred?
Romeo. O blessed, blessed night! I am afeard.
Being in night, all this is but a dream,
Too flattering-sweet to be substantial.
Romeo and Juliet's tragedy in many ways seems to revolve around this priviledging of sweet fantasy over reality - they literally both choose to take themselves out of the only reality we know by removing themselves from life through suicide. Romeo expresses this ambivilence about life/death and dream fantasy/reality in this passage:
Romeo. I dreamt my lady came and found me dead—
Strange dream, that gives a dead man leave
And breathed such life with kisses in my lips,
That I revived, and was an emperor.
Ah me! how sweet is love itself possess'd,
When but love's shadows are so rich in joy!
By choosing the shadow (another word for ghost) of love over actual life, hard and painful, perhaps Mercutio's thoughts on dreams come true for Romeo and Juliet - their dream of love led to the nothingness of death.