One thing Mercutio relates in his Queen Mab monologue is his belief, or feeling, that dreams are really nothing; they are trivial and insignificant, having no deep meaning. We dream about what comes naturally to us and the dreams mean nothing more, as Mercutio explains in his response to Romeo who begs him to stop his Queen Mab speech, saying that he is talking about absolutely nothing. Mercutio says in reply:
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. (I.iv.103-06)
We see evidence that Mercutio is basically claiming that we dream of what comes naturally when he goes on and on from the middle of the speech to describe the different things that different kinds of people dream of once Queen Mab passes over in the night. For example, when she passes over lovers, "they dream of love"; lawyers dream of earning money; ladies dream of kisses; courtiers dream of petitioning the government for some new action; soldiers dream of battle; and parsons dream of "another benefice," meaning of living a different, more free and sinful lifestyle (76-93). From these descriptions, we can conclude that Mercutio is saying that dreamers only dream about what they already know about or what they want. Therefore, Mercutio is expressing his belief, or feeling, that dreams are meaningless beyond expressing what we desire; they cannot prophesy as Romeo believes his dream has.