Serious emotions are easy to turn boring if they're not intense enough. Too much tension can make a reader tired, distracted, or even irritable. But if you combine serious emotions with a touch of humor? You'll have 'em eating out of the palm of your hand!
Examples? Sure! My favorite author, Jim Butcher, tosses humor into his books ALL THE TIME! His protagonist, Harry Dresden, always throws a wise-crack into the dialogue, even when he's staring into the face of certain death. No, scratch that, ESPECIALLY when. Sure, there are plenty of serious emotions that enter into the Dresden Files, and they aren't always interrupted by humor. But just when you think the tension can't grow any tighter, he tosses in a joke. The lets out the tension a tad... just enough for it to get ratcheted even TIGHTER than before!
Next we have Gail Carriger. Her stories are chock full of humor. And it's very witty, too! Her jokes aren't cheap or snarky, they're just a part of the world. There's a level of frivolity in all but the most serious scenes in her books. And it makes what could be very dark stories into light, almost cozy books about an odd woman and the companions she chooses to associate herself with.
Finally, let's talk George R. R. Martin. I'm a newcomer to A Song of Ice and Fire. In fact, I've never read any of his books. I DID listen to the audiobook of A Game of Thrones, though. And even that book, very heavy, deep, serious, etc. has its share of humor. Don't believe me? Just look at the Tyrion Lannister. He always has a joke for the people threatening him.
So what makes you laugh? What books can you think of that have humor in them, even though they aren't comedy or humorous books? Do you like it or hate it? And especially, have you read any books that take themselves too seriously? Do they need jokes in them?