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.