Let's start with the fact that the how-to-train-your-dragon part comes when Hiccup defies the rule of killing dragons and decides to befriend one instead. The whole point of the book was the school that Hiccup had to attend. A school designed to teach vikings how to train dragons, hence the how-to-train-your-dragon part of the book. His heroism comes in taking the weakest dragon and turning it into the strongest by saving the village with it.
There's also Astrid Hofferson, the female student in the school of killing dragons. There was no female students in the book. And, as much as I like the idea of a female viking, I felt as if the movie creators added the female as a love interest to Hiccup and nothing else. As a woman, I don't feel right about that. We are more than just love interests.