Laughing at carrion

I went to the zoo on Friday. Well - more accurately raced around the zoo, chasing various children as they sped past.
Children I'd brought with me, I mean. This isn't that kind of story.
Anyway, collapsing on a bench while the family ransacked the gift shop, I noticed a hyena, grinning at me.
"What are you smiling about?" I said. "All you do is wait around for lions to kill something, then sneak up and eat it while they're asleep. You're basically crows that are too stupid to fly. So why so cheerful?"
"Well," said the hyena, "you've gotta laugh."