I was recently asked the question: “Why do things taste better when they’re free?” While this hardly constitutes a rigorous scientifically testable hypothesis, I thought I’d wax speculative about it for a moment.
The explanation of why some may have this experience lies in the fact that your brain has a unified reward system based on the small molecule dopamine. This system is responsible for many types of associative learning (including classical Conditioning a la Pavlov), and signals rewards to be gained from engaging in various behaviors such as eating, having sex, or doing drugs1. Further, it seems likely that the dopamine system is responsible for making us feel happy about having received monetary returns1.
In the free-food-tastes-better case, this system is double activated. There is a reward for having gotten something for free along with a reward for eating something. However, there is sometimes only one action to assign the reward to: eating. Perhaps when there is no immediate action to be applauded for providing the free-food-prize, the brain simply heaps all the praise on the eating itself, thus resulting in the delightful (and dangerous) experience of the free snack being extra desirable.
1. Caplin, D & Dean M (2008) Dopamine, Reward Prediction Error, and Economics. The Quarterly Journal of Economics. 123(2), 663-701