Based on my experiences on both sides of the screen:
1. 99.9% of games benefit from at least the occasional humorous moment, even the ones where you cry and dress in black. (I can conceive of the 0.01%, but I’ve never played them.)
2. When any group of gamers gets together for a session, laughter is inevitable.
Have you also found both of these laws to be true? Is there a third law (or a fourth, etc.)? Does the first one take place naturally when you GM, or do you need to plan for it?