As an author who's had a lot of experience with reader feedback, I can assure you that what you're talking about is one of the many indescribable elements that separates a true, artistic writer from a poor one. How does a good writer imbue his work with that effect? Well, it's a complex combination of many different things, one of them being pure heart and personality. Realistic and likable characters, riveting conflict, and poetic flow are just a few things that can help to contribute to the effect you're talking about -- but there's others, including one major one. Proper foreshadowing is the best way to bring that sort of smile out of the reader. When you're smiling at someone's story -- or the characters in it -- are you thinking ahead into the story?
It's hard to explain -- a good writer has a knack for the technique, but it comes from creative instinct and not just cognitive prowess.
Hope this answer helped!
-Cook