I've been watching Being Human from The Great Courses. There's a great episode about dopamine. Dopamine is the body's own feel good chemical. It's released when you win a prize or you spot $50 on the ground. Scientists used to think it was triggered by reward but it's not so simple.
It's the anticipation of reward that triggers dopamine. This was discovered using mice. The mice were trained to wait for a light to come on and then press a button to release a treat. It was found that dopamine was released into the brain of the mice when the light came on, not when they got the treat. This explains why gamblers are on a high from the moment they enter a casino not just when they win.
The next thing the experimenters did was to make the treats intermittent. Sometimes the mice got one and sometimes they didn't. This time, when the light came on, the dopamine levels in the mice were even higher. They got more of a high from an intermittent reward than a certainty. Even if the treat was only given every 10 presses of the button, the mice still got more of a high than if they got a treat every time, but the highest levels of dopamine were recorded when the button delivered a treat 50% of the time.
Casinos know this. In order to keep their customers at the optimal high, they have to convince them that their chances of winning are around 50%. They do this by making the customer feel like a winner before they even start to gamble. They give them a room upgrade when they check-in, offer free drinks and pay pretty girls to flirt with them. Once the customer feels like they're in luck, they're much more likely to bet.
When it comes to boy meets girl the same applies. We get most dopamine from the anticipation of a possible reward (rather than certainty) hence playing hard to get, or being unavailable, can increase someone's attractiveness. It also explains 'the thrill of the chase' i.e. why people lose interest once they get what they want.
Finally, this research confirms that it's easier to avoid temptation than to overcome it. Once you have what you want in sight, the enjoyment has already started. So better to stay out of the casino, patisserie or car showroom than to be 'just looking'!