What Makes Literature Literature?

Last week, I reread one of my favorite books in the world: Joy in the Morning. It's by Betty Smith, better known as the author of A Tree Grown in Brooklyn (and another favorite). Both books are just so rich and layered, filled with interesting characters and real emotions. 

Right after finishing Joy in the Morning, I went back to reading commercial teen fiction, which has been my focus all month (except for The Christmas Village mentioned in my previous post, which doesn't fit into that category). Anyway, up till then, I'd been enjoying commercial teen fiction, but suddenly it paled in comparison. It wasn't literature. And literature was somehow better. But I can't put my finger on what the difference is. 

I've been thinking about it a lot lately. Both literature and commercial fiction can offer interesting plots, well-rounded characters, a real understanding of the human condition, etc. I still respect commercial fiction authors immensely and I do enjoy their books. But I want to know: What is it that makes literature literature? 

Your thoughts welcome!