Being desensitised or oversensitized to various aspects of our own chemical control systems will drive our behaviour and emotions.

Dopamine Pathways. In the brain, dopamine plays an important role in the regulation of reward and movement. As part of the reward pathway, dopamine is manufactured in nerve cell bodies located within the ventral tegmental area (VTA) and is released in the nucleus accumbens and the prefrontal cortex. Its motor functions are linked to a separate pathway, with cell bodies in the substantia nigra that manufacture and release dopamine into the striatum. (Photo credit: Wikipedia)

One way to think about our own brain, personality, and physiology is to consider our sensitivity levels to various neurotransmitters and hormones.  Being desensitized or oversensitized to various aspects of our own chemical control systems will drive our behavior and emotions.  This happens whether we’re aware of it or not, so we might as well try to understand what’s going on in our brains and endocrine systems.
Most people who live with artificial light, electronic devices, internet connections, abundant food, processed foods, and other conveniences of modern life will eventually experience some degree of being “out-of-whack” in terms of neurotransmitter and hormone sensitivity.

For example, too many high-glycemic meals (high in sugar and/or starch) can lead to insulin resistance.  The pancreas is constantly secreting insulin to keep blood sugar levels in check, and cellular tissues throughout the body become less sensitive to its effects.  The result can be Type 2 diabetes, a devastating condition which progressively destroys the body and shortens lifespan.
In a similar way, there is some evidence that the brain can become resistant to serotonin, resulting in depression.  What causes chronically elevated serotonin?  Once again, high-glycemic meals may be a culprit, though the chemistry is complicated and much disputed.  Artificial light in the evenings may also keep brain serotonin levels elevated, by preventing the conversion of serotonin to melatonin (melatonin helps regulate our circadian rhythms, and without it we won’t feel sleepy).
Dopamine resistance is also well-documented.  Stimuli that trigger our reward centers will release dopamine, which is related to feelings of excitement and anticipation.  Think Charlie Sheen — cocaine and hookers — and you’ve got a recipe for dopamine resistance.  The same brain chemistry can be achieved (to a lesser extent) via dedicated use of videogames, political news, porn, gambling, or any of the myriad of things we can become addicted to.
Reduced sensitivity levels to various hormones and neurotransmitters have a variety of effects, but none of them are good.  Fat, depressed, bored, numb, anxious, irritable … the list goes on.
So how do we increase our sensitivity to our own chemical control systems?
In some cases, less stimulation works.  With less sugar and starch, our circulating insulin quickly drops, and our cells become more sensitive to the effects of insulin.
In the case of dopamine, exercise can increase both dopamine production and receptor sites.  If we’re experiencing problems with motivation and excitement about life, exercise may provide a fix.
In the case of serotonin and melatonin, I experienced profound results from going a month without electric light.  As inconvenient as it was, the experiment had a shocking effect on my psychological state.  My takeaway was to keep the lights lower in the evening, turn them off sooner, and use f.lux on my computer.
Addiction and Brain Modeling
One essay I keep coming back to is Paul Graham’s The Acceleration of Addictiveness.  Graham is on to something here — a real Big Idea.  We discover and create substances and entertainment media that super-stimulate our evolutionary reward centers (via sensory inputs and/or chemical triggers).  Refined sugar, alcohol, cigarettes, porn, videogames, gambling, alcohol, cocaine — they all trigger chemical cascades that make us feel good in the moment but have costs later on.  This cycle of addictive-thing-creation is speeding up, but we’re not coming up with ways to “inoculate” ourselves against addiction any faster.
So how do we get addicted to stuff?  The general pattern is that the stimulus triggers oversecretion and/or overproduction of a neurotransmitter or hormone.  To maintain homeostasis, the nervous system and/or endocrine system reduces the number (or in some cases sensitivity) of receptor sites.  Whatever the hormone or neurotransmitter is supposed to do, it does less of.  We become tolerant of the substance or behavior in question.  We need more of the same stimulus to get the same feeling or physiological effect.
This is a vast oversimplification.  Cocaine and meth are both stimulants, but they affect the dopamine system in different ways.  Cocaine prevents the reuptake of dopamine, while amphetamines cause more dopamine to be secreted.  The end result is similar — too much dopamine (and neurotransmitter by-products of dopamine) bouncing around the brain.  This leads to excitement and feelings of anticipation in the short-term, but severe dips in energy and motivation later on. The brain becomes less sensitive to dopamine as a way to protect itself and restore balance.  When the chemical stimulus is removed, the brain is still desensitized to dopamine and needs more to feel “normal.”
Behavior can densensitize even a drug-free brain.  Risky behaviors, pastimes, and careers (e.g. extreme sports, high-stakes finance, or professional gambling) can model the brain in powerful ways.  So can intense, life-changing experiences.  A brain that has been to war is not the same as a brain that has not been to war. Some people are attracted to novelty and excitement because of genetic factors that affect brain structure and neurophysiology, but behavior models our brains above and beyond the effects of genetics.

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