How is Romeo impetuous?

Romeo's impetuosity can be seen in the astonishing ease with which he falls in love. It's a standing joke among his friends—especially Mercutio—who rib him mercilessly over it. It helps to explain why they find it so hard to accept Romeo's love for Juliet; they think she's just another brief infatuation like Rosaline.
But this time it's the real thing, as Romeo shows—somewhat ironically—by his impetuosity in the famous balcony scene. There's something about Romeo's impetuosity here that stands apart from previous instances. One certainly can't imagine him serenading Rosaline in this way. While Romeo is declaring his undying love for Juliet, using the most flowery romantic language imaginable, Juliet, though no less smitten, is much more grounded, trying to get Romeo to see the dangerous consequences of the two young love-birds going against the wishes of their warring families.

I would say the height of Romeo's impetuousness is contained within act 5, scene 3, when he commits suicide. Juliet took a sleeping potion in order to fake her death and Romeo believes she is actually dead. He immediately goes to the apothecary to acquire poison, then returns to Juliet's tomb. Romeo kills himself in order to join Juliet in death. Romeo and Juliet have only known each other for a short time period. Their love progresses at a shocking pace and Romeo chooses to give up the entire rest of his life for Juliet. Juliet wakes up shortly after Romeo poisons himself, then stabs herself with a dagger. Had Romeo not acted so rashly, they would have both lived. Given this ending, Romeo's impetuousness is his tragic flaw and leads to his (and Juliet's) downfall.

Romeo had to be characterized as unusually impetuous, even for a young man. Everything in the play has to happen within a limited time frame, and Romeo is the instigator. He sees and falls madly in love with Juliet in a matter of minutes--or seconds. He also forgets Rosaline within a short time. Then he arranges to be married to Juliet with the same impetuous haste. By Act III, Scene 5, they have already spent the night in each other's arms. Romeo's impetuosity seems infectious. It influences Juliet to marry Romeo without any engagement period and without her parents' knowledge, much less their consent. It also influences Friar Laurence to perform the wedding immediately and in secret. Perhaps Shakespeare decided to make Juliet such a young girl to make it plausible that she could be so easily swept away. A lot happens in Romeo and Juliet within a very short time frame. It seems as if they have lived out their whole lifetimes on "fast forward," so to speak.