In act 1, scene 4 of Romeo and Juliet, Romeo and his friends are traveling to the Capulet’s party, and Romeo is bringing everyone down. He is continuing to harp on the fact that Rosaline doesn't love him back. Rosaline, by the way, is choosing to become a nun. His friend, Mercutio, tells him about why he is going so crazy: he tells him that the love he feels is the madness of Queen Mab, a fairy that drives people crazy. Mercutio explains,
Through lovers’ brains, and then they dream of love;
O’er courtiers’ knees, that dream on cur’sies straight;
O’er lawyers’ fingers, who straight dream on fees;
O’er ladies’ lips, who straight on kisses dream,
Which oft the angry Mab with blisters plagues,
Because their breath with sweetmeats tainted are.
Sometime she gallops o’er a courtier’s nose,
And then dreams he of smelling out a suit
The imagery that Shakespeare uses paints Queen Mab to drive people mad with desire; instead of ordinary “romantic love,” the love that Queen Mab would push someone to is more like lust or obsession—the same urging that pushes lawyers to dream of fees or soldiers to dream of slitting throats pushes lovers to dream of love.
The reason Shakespeare uses this mad type of love is explained when Friar Lawrence when he says,
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume. (act 2, scene 6)
The love that Shakespeare describes in Romeo and Juliet is a dangerous, fanatical love. It is like powder primed and ready to explode, which is the reason that the two lovers are pushed to suicide, the most dramatic and unchangeable decision. If they cannot be together, they would rather be dead—that sounds exactly like what Mercutio describes when talking about Queen Mab.