I read books because I like to read. Why? For the same reason I like to play video games. I don't know.
Finding depth in books is not something a lot of people care about, but I try to do it. My problem with literature is that often, it points to something blatantly. "Look!" it cries. "This is my message! Understand it!" But that's not right. You shouldn't need to understand deep meaning in a book to enjoy it. And understanding it shouldn't be a shocking revelation, either. It should be a quiet realization.
A wise man once said, "If you want to tell a message, send a telegram." I believe that he was right, and he still is. (I have no clue who said that, sorry.) A book created for the sole purpose of telling us something is unnecessary and stupid. To me, a good book is one that tells us about our world. It's a story about us, our planet, our nature, and the things that happen here. If you have all of these things, meaning will naturally arise, just like it does in the real world. A good book helps us view the universe from a new angle. People can choose to take whatever meaning they want from a book like this, go as deep as they want. And, if they're shallow people, they probably won't want to really be deep. That's fine.
It's a little like the difference between putting someone on the beach and dropping them into the ocean from a helicopter, I think. But maybe that's just because I'm using the word "deep" too much.