You misinterpret my conclusion. It's important to be happy because psychologically (because of the way our brain is wired and evolved over thousands of years), we must seek happiness. Even if it doesn't matter after you die, why would you want to live an unhappy life? However, the same conclusion doesn't apply to spending time on "worthwhile" activities. Evolution per se doesn't care if we strive to write a new compiler, go to professional conferences or work hard at a bank earning millions. All it cares about (apart from our reproduction) is our happiness.
Apparently, happiness is also linked to reproduction and many "worthwhile" activities are ultimately linked to happiness. But I'm digressing here... Brain seeks happiness and in absence creates discomfort that is by definition undesirable and that is why (so obviously) it is important to be happy even if "nothing really matters" ultimately.
> All it cares about (apart from our reproduction) is our happiness.
This isn't quite true. All evolution cares about is reproduction, that's literally it. It only tangentially cares about happiness/earning millions/whatever if it improves reproductive ability (which they probably do).
Apparently, happiness is also linked to reproduction and many "worthwhile" activities are ultimately linked to happiness. But I'm digressing here... Brain seeks happiness and in absence creates discomfort that is by definition undesirable and that is why (so obviously) it is important to be happy even if "nothing really matters" ultimately.