We need time to relax and enjoy ourselves too, no matter how dark the story. The occasional humorous moment helps with that. That's one of the things that Joss Whedon excels at in his storytelling.
Patrick RothfussAll stories are true," Skarpi said. "But this one really happened, if that's what you mean.
Patrick RothfussAlso, worldbuilding touches all aspects of your story. It touches plot and character as well. If you don't know the culture your character comes from, how can you know what he's really like? You must know your characters on a much deeper level than you would if you just shrugged your way into a cookie cutter fantasy world.
Patrick Rothfuss