We're not happy because we stopped evolving 10,000 years ago.

Humans stopped evolving about 10,000 years ago. That's when civilization began and permanently changed our lifestyles. We went from hunter-gatherers living on the savanna, to farmers.

The change happened quickly and genetic traits that would make us more successful (good with animals? good at identifying and growing different types of grasses?) began to be selected for.

Except that human evolution is slow, with approximately 50 years between generations. Culture changed again. We became an industrial society and now traits like machine aptitude, fine motor skill dexterity became selected for.

Except that culture changed again and now we live in a post-industrial society. Now those other traits don't matter because most of us work in the service industry. Basically things change too fast for our bodies to adapt. That's why we've stopped evolving.

No genetic traits can become dominant because they don't have enough time to spread through our populations. Things change too fast.

This is why we aren't happy. Our bodies are living 10,000 years behind our minds.

(I got the evolution part from the book Why Beautiful People Have More Daughters by Alan Miller. I made up the part about happiness, just because it sounds cool.)