One of the main reasons -- well, besides the Christopher Walken MIND CONTROL thing:
"...laughter is the cure for everything. A lot of people go through the whole day without laughing. It’s such a great cure because it elevates all the stress you have. It reminds you that everything is temporary. If you can get your mind around the fact that everything that is temporary, what more do you want? Everything is temporary. Nothing stays the same, everything changes. There is no point in going over the same thing over and over again."