I think that sometimes I get so caught up in a book, so emotionally invested in the characters that when something big happens in the story I am emotionally affected by it as well.
Piers Anthony, who many of you may know for his young adult work has a nice little series called "Incarnations of Immortality" series, and while it isn't entirely whimsical it has plenty of whimsy. Each book is based on the personal incarnation of an eternal concept; death, war, time, nature, and fate and deals with issues like sex, sin, martyrdom and philosophy. They're quite good and realtively light for the topics which are addressed in each book. Though, whimsical they are all very nuanced and different from his Xanth works.
But let's not forget:
- Ready Player One by Ernest Clines
- Redshirts by John Scalzi
- Year Zero by Rob Reid
While not sci-fi "Guards! Guards" by Terry Pratchett will always bring a smile to my face.