Getting satisfaction
Think of Grand Central Terminal in New York. All the trains represent potential things that you might do, but all the trains are only going through the terminal once. So you can only do one thing at a time, just like we only have one body to dedicate to a task. You need a mechanism for deciding what to do, for switching the tracks of the trains that are constantly vying for access to your physical body. The striatum controls the trains, decides which destination, and dopamine is the signal to switch tracks.
If you do something at which you are highly practiced, then you have little opportunity to encounter something novel or unexpected, so dopamine and satisfaction may be low. But when you do something that takes you beyond what you have done before, you are in unknown territory and novel information will flow into your striatum, pumping out dopamine, which in turn forces you to act on the information. The release of dopamine in response to the novel information is the essence of a satisfying experience.
We have all been taught that stress is not a good thing, and we should try to minimize it because it can lead to heart disease and hypertension and even depression. But what you found seems to be counterintuitive.
Of course it is important to keep levels of stress manageable, but it is not as simple as saying stress is bad. In fact, some stress is good because it releases a very important hormone called cortisol. While dopamine is a neurotransmitter, cortisol is a hormone that is produced in the adrenal glands on top of the kidneys. It is known that cortisol is released in response to mental, physical, and psychological stress, and it has been thought of in a negative way. But the paradox with cortisol is that it can do many positive things as well. Without it, we wouldn't be able to survive at all. When it gears up the autonomic nervous system to help us deal with stress, it activates the body in a general way so it can run or fight or do whatever you can do to deal with the stress. Cortisol goes up, for example, after exercise. But with dopamine in the striatum, cortisol interacts synergistically. Novelty releases dopamine, stress releases cortisol, and when they come together they create an intense feeling of satisfaction.
Is there an evolutionary angle to this?
The old evolutionary model that the brain exists to help us survive and reproduce is fine but too broad. If you go beyond that, the thing that helps us to survive and reproduce is the capacity to adapt in the world and to learn. The world is so unpredictable; most animals have brains like sponges absorbing so much in their drive to learn. Our brains are just primed for novelty. Nature never said you had to be happy. It said you had to learn to adapt to the world.
But if novelty and stress are so great, why do we shy away from them?
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