It seems to me that humour is everybody's way of keeping sane and standing off from the situations so that they can see it intellectually, as well as emotionally, and I don't know whether you've noticed, but if somebody tells a joke, it's nearly always a mini fantasy.
“There's no subtext in Harry Potter really; it's all magic, anything can happen. Why do I say this? Because it's a magic spell. It's quite nice in a way. There is a real freedom to it.”