Who doesn't like a joke? What about a funny TV show or movie? I think the same for books that have funny characters. You just can't put the book down because you are so engaged with the storyline and the characters. I love when I laugh out loud while reading and my kids say, "What's so funny?"
I think authors who are able to convey those happy emotions are very talented. Susan Elizabeth Phillips comes to mind, as well as Sophie Kinsella, and Emily Giffin.
What book has made you laugh out loud?