A simple model separates every human in his consciousness and his subconsciousness.
Fun is a carrot the subconsciousness uses to make the consciousness do something. It is a carrot the consciousness wants to eat. But every time the consciousness is about to reach the carrot the subconsciousness moves it a bit further away.
The consciousness tries everything to make the subconsciousness give it the carrot. And games are just one of the tools the consciousness uses to try and trick the subconsciousness into giving the carrot. But the subconsciousness isn't stupid, either, and ultimately has all the weapons it needs. That's why games - or any activity really - don't ever make us perfectly happy. There's always something else we want.
Once we got the car we want the house. Once we got the house we want the neighbor's house. Once we got the village we want to be elected President and once we are President we want to leave our footprint in history. And this footprint can never be big enough.
There is one way to circumvent the subconsciousness: drugs which tell your brain to be happy now, no matter what. On the one hand, effective drugs work beyond anyone's wildest dreams. On the other hand, when looking at drug-addicts we, that is our consciousness, understand that at the end of the day the subconsciousness acts in 'our' best interest when it pushes us forward all the time.
Even though we want to reach the carrot it is the journey towards it that counts because this journey is where reproduction of the 'fittest' takes place. The journey is a way to determine who is better able to reproduce and to allow him to actually do it. Contraception is probably the biggest victory the consciousness ever had over the subconsciousness.
At this point I am inclined to venture into the reason for two sexes, but that's not possible within a blog post. As usual, the Wikipedia page and all the links are a wonderful place to read and understand.
This is getting quite removed from game design here. With the next post I hope to get back on track ;)