Romeo and Juliet is filled with textual examples of Romeo and Juliet's impulsiveness.
1. "To smooth that rough touch with a tender kiss" (I, v, 96). This line, spoken by Romeo, speaks to the impulsive nature of his love for Juliet. The line is spoken at their first true physical meeting and Romeo is already in love, deeply, with Juliet. This impulsiveness is compounded by the fact that Romeo has already forgotten Rosaline.
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
Ere one can say "It lightens" (II, ii, 117-120).
Here, while Juliet recognizes the impulsiveness of her love for Romeo ("too rash, too unadvised, too sudden"), it does not deter her from her love for Romeo. Instead, she later asks Romeo to consider marriage.
3. "Thy purpose marriage, send me word tomorrow" (II, ii, 144). Here, Juliet proves to be just as impulsive as Romeo. She is moved by his honorable love and immediately wishes to marry him. Therefore, she does not wish to wait long, or give Romeo time to really consider the proposal, to marry.
4. "Wisely and slow: they stumble that run fast" (II, iii, 94). This line is spoken by Friar Laurence to Romeo. Romeo is telling the friar that he must move, "stand on sudden haste" (which also speaks to impulsive behavior). The friar is warning Romeo that impulsiveness will lead to one's downfall and, therefore, speaks to the tragic nature of both the play and foreshadows the deaths of both Romeo and Juliet.