Hiccup is both the main character and the narrator. He is the one who introduces us to the movie's setting and society. Also from his introduction, we sense his tone of indifference and the movie's humor. In the following clip, we get more of both:
My favorite line in that clip is the last one about keeping in all that Viking. Gesturing at Hiccup's body as the problem highlights his lack of strength. However, Hiccup's body is not the only reason he cannot convince others--and us the audience--of his Viking fury. His tone does not sound confident of or even excited in what he states. He is indifferent to this lifestyle and not even his own words can convince himself to follow it. We are shown this factor right off the bat to understand the story's direction.
Something else to notice about Hiccup's voice is that it's matured. Hiccup is a goofy adolescent who has a crush on a girl. However, his voice is not a boy's or changing from puberty. The voice actor is an adult not a boy. Thus, Hiccup carries seriousness along with the humor of being an adolescent. His voice has matured and he is trying to be mature himself. One of the story's themes is coming of age then; He is finding himself and his place in society.
Hiccup's voice is heard before we see the society and dragons so I thought it was worth looking at.