Evolution seems to explain most of it. There's humanity as species, where decision making process is quite primitive, hormones drive most of it. And then there's humanity as civilization, where decisions are supposedly driven by culture and obtained knowledge. For millions of years hormones were the only tool for decision making and evolution was the only tool at our disposal for progress.
But now those two are at conflict, and the winner is clear. We are way past those times where evolution has any noticeable effect on our species. More has changed for humanity in the last 100 years than in 50,000 years somewhere in the middle of our history.
With this mindset, treating happiness as the ultimate goal seems ancient and barbaric. We're not cavemen anymore, it's no longer cool.
That said, it does give an easy meaning to those who can't choose the meaning themselves. If you don't have any purpose in life, happiness is the inherited factory default setting.
I decided I don't want to be a boring human being with factory default settings. I'd rather be wrong than boring.
