Romeo appears to be very impulsive in love. This love-at-first sight approach, given his age, isn't odd, however. His switching his affections from Rosaline to Juliet, is certainly very speedy and impulsive, though.
He's impulsive in his decision to jump the Capulets' wall to avoid his friends, which puts him in his enemy's garden just under Juliet's balcony. This impulsive act leads to their declarations of love and pledge to marry, an impulsive thing to do on the eve of meeting someone.
He's also impulsive in his engagement of Tybalt after Mercutio is killed. He enters the scene determined to be a peace-maker, but lets his anger and grief get the best of him, and he murders Tybalt. Once he realizes what he has done, he chides his own impulsiveness with the words, "I am fortune's fool."
He impulsively reacts to the "news" that Juliet is dead by flying to her side to kill himself. He reacts before he can get word from the Friar that Juliet only pretends to be dead.
And, finally, he impulsively kills Paris when he meets him in the Capulet tomb. He simply kills him because he is in his way.
Overall, impulsiveness is a characteristic that defines Romeo's behavior throughout the events of the play.
Romeo is endlessly the impulsive teenager, caught up in the intense whirlwind of whatever his emotions at the moment happen to be. He transfers his love from Rosaline to Juliet in a flash and scales a high orchard wall to seek out Juliet the same night he meets her—despite the risk of being killed if her male relatives find him there. Romeo agrees to marry her almost immediately, imprudently murders Tybalt out of passion, and quickly commits suicide when he thinks Juliet is dead rather than waiting a moment or seeking out more information. Being young, Romeo habitually acts first and thinks later.
Friar Laurence tries to counsel Romeo to slow down and not allow his passion to burn at such a high intensity, telling him such a love will never last, but his words fall on deaf ears. Though the play locates the tragedy in the way innocent young people become the victims of a senseless feud, Romeo's headlong impetuosity arguably helps to propel the couple towards a bad end.