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 RothfussAfter an awkward pause, Bast extended his hand. Chronicler hesitated for a bare moment before reaching out quickly, as if he were sticking his hand into a fire. Nothing happened, both of them seemed moderately surprised. "Amazing, isn't it?" Kvothe addressed them bitingly. "Five fingers and flesh with blood beneath. One could almost believe that on the other end of that hand lay a person of some sort.
Patrick RothfussThe air was heavy with the smell of leather and dust, of old parchment and binding glue. It smelled of secrets.
Patrick Rothfuss