"Don't worry about it." Hiccup said, waving Hal off, "I know when to cut myself off. Habit really. I've been in charge of the three craziest people on my island since I was 15. It doesn't go well when I let them talk me into things sober, I don't want to know what would happen if I was drunk."

Hiccup was fully willing to do crazy things on a whim, but it was usually tempered by common sense. Drunk him would very likely lose that sense.