Happiness Maximum by Kally Kislowicz
There is a Happiness Maximum in my family. This means that at any given moment the available happiness is divided, often unequally, amongst the six of us. The parameters of the equation make it all but impossible for each of us to be feeling entirely happy at the same time. This explains the Bad Mood Principle which accounts for the fact that no matter how much fun we are having, one person will always be flirting with, if not entirely subsumed by, a bad mood.