A first impression goes a long way. That's true with people, and I think it's also true with books. When I'm choosing a new book, once I get past the title and the author, it's the first line--or first few lines--that make the decision for me.
When I was a kid, my mother used to tell me that when she had a good book to read, it was like having a good friend. It took me a long time to understand what this meant. I've learned it doesn't happen all the time, but when it does, it can be magical.