When you read a book, you don't usually want the hero/heroine to constantly whine and moan about what they have to do, so why should you write them that way?
A hero is defined as a person who, in the opinion of others, has heroic qualities or has performed a heroic act and is regarded as a model or ideal or a man of distinguished courage or ability, admired for his brave deeds and noble qualities. A heroine is the same thing, just in female form.
For the purpose of this post, I'll be talking about heroines, since most YA books I've read have female narrators. But all of this can be applied to a hero, too, for the most part.
In my opinion, when you're writing a character you want to be the heroine, who is mainly going to be the narrator, you still need to make them realistic. Sure, a heroine needs to have admirable qualities, but at the same time, they still need flaws. They can't be all high-and-mighty; they need to have problems. To me, that's what makes a true heroine - someone who can recognize they're not perfect, has some major issues just like everyone else, but at the same time still goes above and beyond to help other people.
Another thing I think you have to be careful of is that your heroine can't just become a heroine right out of the blue. She can't be completely boring and whiny and teenager-like and then suddenly, at the climax of your story, save everyone. Even before she can be recognized as a "true heroine," she needs to have admirable qualities. Even if it's just something like stopping some bullying in the hallway.
Which brings me to another point - heroes and heroines can be found anywhere in your story. It doesn't have to be the narrator/main character. Sometimes, it's the random secondary character that saves the day, and I think that's okay - as long as there's some kind of predecessor to it. Maybe the secondary character stopped someone from bullying your main character. Maybe the secondary character stopped your main character from being chowed on by a wolf. I dunno. It can be anything. o.o
Since this post is so scrambled and probably confusing, I'll just say my main point is this: Heroes and heroines can be any character. For the most part, they're just like the other people in your book, but you need to make sure they're not the stereotypical whiny teenager. Of course, that goes for any character, really, but when it comes down to it, your heroine has to save the day - and hopefully do it in a smart way.