After he realizes what he has done he exclaims "O, I am fortune's fool" (3.1.135)! He understands that even though he has just wed Juliet he may be put to death for killing Tybalt. .
Juliet, although not as impetuous as Romeo, says and does some rash things as well. Right after meeting Romeo at the masquerade party, she says to her nurse "Go, ask his name – If he be married, my grave is like to be my wedding bed" (1.5.148). This means she would sooner die rather than marry anyone else other than him. Later that night in the garden Romeo and Juliet declare their love for each other. Juliet is the one to ask Romeo to marry her even though they have just met a couple of hours before. She says "If that thy bent of love be honourable, Thy purpose marriage, send me word to-morrow" (11.2.143). So in spite of their families being enemies they love each other and are determined to be together. .
Friar Laurence is impetuous at well. He marries Romeo and Juliet, even though they just met and the day before Romeo was in love with Rosaline. He marries them because he hopes that it will stop the Montague and Capulet feud if the two families at brought together by marriage. He says, " In one respect I'll they assistant be; for this alliance may so happy prove to turn your household's rancor to pure love" (II.3.90).
After Romeo is exiled for killing Tybalt, Juliet is overcome with sadness at the thought of never seeing Romeo again. Lord Capulet thinks she is grieving for her dead cousin and gives Paris permission to marry Juliet hoping it will cheer her mood. Juliet is forced to agree to marry Paris. Juliet thinks the only way to get out of marrying Paris is to die. She asks the Friar to help get her out the marriage to Paris. She says she would sooner be dead than marry Paris. Friar comes up with the idea of drinking the potion that will make her seem dead.