What does Juliet mean when she says, "I have no joy of this contract tonight./It is too rash, too unadvised, too sudden;/Too like the lightning, which doth cease to be"?

These lines are spoken in Act II, Scene 2 of Shakespeare's Romeo and Juliet. The scene is usually referred to as the balcony scene as Romeo has gone into the Capulet orchard to catch another glimpse of Juliet, and the scene plays out with Romeo standing below the balcony speaking with Juliet. He just met Juliet at the party in Act I and has already fallen head over heels in love with the girl. Likewise, Juliet has fallen instantly in love with Romeo, but here she expresses her misgivings over the speed of their courtship. Because she is a Capulet and Romeo a Montague there are problems because their families are mortal enemies.
At the end of the party in Act I, Scene 1, the two have discovered each other's identities, but this does not stop Romeo from pursuing his emotions and his encounter with Juliet at the balcony confirms his suspicions that Juliet shares his feelings. Unlike Romeo, Juliet is reticent to engage in something that would be forbidden by her parents and family. The relationship is "too sudden" and she realizes that it may lead to trouble. The audience is already well aware of the enmity between the two families as the play opens with a street brawl instigated by the servants of Juliet's family and her cousin Tybalt. When Romeo appears below her balcony, she is instantly aware that his presence could mean his death because of the hatred her relatives have for the Montagues. Ultimately, however, she cannot hide her feelings and she pledges her love for Romeo, despite her reservations about the hasty nature of their love.

I have no joy of this contract to-night:It is too rash, too unadvised, too sudden;Too like the lightning, which doth cease to be