Not that I've been diagnosed with anything terminal lately, or any other sort of desease, but I would think that laughter, like being intoxicated (for me) it's just sort of an excape from pain, whether it be physical or emotional. I think it's like for that time nothing else matters, you're happy for that split second, until you start thinking again.
Then again, thats just my opinion, I can't really say I'm happy all the time, rather quite the oppostie, I'm what most would call "emo", though I find it very hard to smile at times, but laughing just stops everything, and now I'm repeating myself, sorry y'all.