I'd imagine, although I'm not entirely sure, for a similar reason why a lightbulb won't always be on. Something needs to power it, and without the power it can

I'd imagine, although I'm not entirely sure, for a similar reason why a lightbulb won't always be on. Something needs to power it, and without the power it can sit in the off-state for eternity. If the power stops, the light stops. With happiness, something needs to start it, and without anything to keep it going, the happiness goes away. You could also say "Evolutionary Reasons," although that's much less interesting and more of a cop-out. The argument on that front is just that sadness is the optimal state of a human being to succeed, whilst happiness is the way to reward them for succeeding. If they're always happy they'll change nothing. It makes sense, obviously, but it's very much a low-hanging fruit.