I just read Howl's Moving Castle by Diana Wynne Jones, and plot-wise it is not a perfect book. Perhaps it was just me, but it got a little convoluted and confusing, but still, I could not put it down, and I think it is a book I will reread once a year or so.
Why? Because I loved Sophie. And I loved Howl. And I loved what happens to them, even though I didn't understand it all. I loved the fire demon too and the poor assistant. Their emotional reactions and their attitudes were believable and relatable, and I could put myself in their shoes even though the world they live in made no real sense compared with mine.
The same is true of the How to Train Your Dragon series. I love it for it's humor, and I do enjoy the plot, but mostly I just love Hiccup. What brilliance on the part of Cressida Cowell. She created a wimpy, sympathetic character in the midst of barbarians, so of course you side with Hiccup. You want him to succeed because he is so different from the people around him. Much like Harry Potter. Harry is so different from the Dursley's, he has your immediate sympathy. But if you put a rather wimpy character in the midst of normal people, or above average people, not so appealing. Fishlegs, Hiccups only friend, is also a brilliant character. I just read Shaemus a scene in How to Cheat a Dragon's Curse where Fishlegs is barreling down a mountain side on skis he should not even attempt to wear to confront the most brutish Vikings on earth—all because they have killed a stag. The pictures are hysterical—crossed skis, flying poles, Fishleg's twisted helmet—but Fishlegs has our sympathy the entire time. We are ROOTING for him.
And that's what we want, right? To be rooting for our characters.
Just like we want others to root for us.