Why is sex fun?
I think that's one of the easiest questions to answer.
Go back 500,000 years, and split proto-humanity into three groups: the first is "Sex? Yeah!", the second is "Sex? Meh", and the third is "Sex? Ew". Now I can tell you now which of those groups is going to have the most babies!
Or put it another way. Sex is hereditary: if your parents don't have any children, nor will you.
It's actually a fun exercise to identify what people like, and then try to work out why that would be advantageous.
Water, for instance: people love water! If there's a lake in your town, then people will flock to it, and on nice days spend hours just hanging out there. I suspect it's why Dutch people are amongst the happiest in the world: there's just a lot of water. Children love it: give them access to a little paddling pool full of water, or to a hose, and you won't have to entertain them for the next few hours. I remember driving through England, passing through a wood, and my children in the back saw a little winding stream and yelled "Stop! Stop!" and since we weren't in a hurry, we did. Well, the boys kept themselves amused for ages, throwing stones in the water, racing sticks, splashing. In the end we had to drag them away, because we did have a destination to reach.
Or heights! You may know that in Paris there a hill, Montmartre, with a beautiful white church at the top, the Basilica of the Sacre Coeur. Visitors flock to it, spend 15 minutes admiring the inside of the church, and then sit for an hour on the steps outside, just looking at the view. Sometimes those steps are so packed, it's hard to get down them.
But there's one thing that humans love that is harder to explain, because while humans love it, every other creature on earth is terrified of it: Fire. Go back a million years and split all creatures into three groups: "Fire? Run!", "Fire? Meh" and "Fire? Yeah!", and see which of those groups produces more babies!
Of course, fire had advantages: it kept us warm, it frightened off predatory animals, it allowed us to cook our food. While it's easy to see why fire is useful for humans, it's harder to see how they got from "Fire? Aargh!" to "Fire? Yeah!". When I first arrived in Amsterdam, there was a large fire near me: the crowds of people who went to look were enormous!
So let me tell you a story. A colleague/friend of mine was baby sitting for another colleague/friend. The child in question was in the pre-walking, pre-talking stage, where its only method of communication is by crying. And that evening she was communicating! And no matter what my friend did, she just would not stop. Finally in desperation, he decided to take her for a walk, and loaded her in one of those front-loading strap-on carriers, with her facing forwards, and he walked down the street.
At the end of the street was a vending machine, possibly a cigarette machine, which had a single LED light on it to show that it was working. When she saw that light she stopped crying! She was fascinated by that light, and kept reaching out to touch it, and kept staring at it.
When my friend told me this story, a light went on in my head. The thing about LED lights is that they flicker. They flicker so fast that you normally don't notice it, but if you wave one around, you will see a series of dots rather than a blurred line.
What I realised was that humans have a flicker gene! If something flickers, it stimulates some small pleasure centre in our heads, and makes us happy. This explains a lot of our activities. Why we are willing to sit around a camp fire for hours on end, telling each other stories, or just staring into the fire. It probably explains why we like diamonds, and glitter.
A hundred years ago, every house had a sitting room with a hearth with chairs circled around it: that was where people sat and read, or knitted or sewed, or just talked, for hours on end.
And with the introduction of central heating when the fires were no longer used, what replaced them? Televisions! And what do televisions do? Flicker! Faster than we can see, but they still stimulate our flicker gene pleasure centre. This would explain why people are so willing to watch the TV for hours on end, even if it's all crap.
And it also explains why we love looking at our phones so much: not because there's anything particularly interesting there, but because just looking at it gives us a little bit of pleasure.
So what we can see is that a gene doesn't have to be advantageous in itself, only its effects.
My theory: if you had a phone that didn't flicker, you wouldn't look at it nearly as much.