What is so Funny?

One line from the Simpson’s Movie that made me laugh was when Bart says “Dad, this is the worst day of my life!” to which Homer replies “Worst day <dramatic pause> so far!”.  If Bart had said “Dad, this is the best day of my life!” and Homer had replied “Best day <dramatic pause> so far!” it would not have been funny – it would have sounded cheesy. Why is that?  What does this tell us about how we can sometimes confuse humour and pleasure?  If we laugh when we are unexpectedly confronted with someone else’s emotional distress (the basis of slapstick humour); and we also laugh when we see other people laughing; and we also laugh when we have our expectations exceeded (the basis of surprise parties) then by simple association of the feeling (pleasure) with the behaviour (laughter) we have a recipe for collectively laughing at someone else’s distress.  More sinister is that we can unconsciously plan to derive laughter (and by association pleasure) by deliberately engineering distress for others.  Humour, like any process, can become sick.