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If I consume certain external chemicals (in food, drink, pills, injectables, powder, whatever), then those chemicals and their reaction byproducts find their way to my brain and alter the local neurotransmitter chemistry at the synapse level. Maybe the result is that I feel Good, or maybe not, but what I actually feel is the effects of the local brain chemistry changes caused by the external chemicals I added, not the external chemicals themselves. That's all our brains understand, and all drug effects come from certain kinds of biochemical changes they produce - which is why drugs can be roughly categorized according to which neurotransmitters they affect, where, and how, and why some very chemically-different drugs can have similar effects.
Activities can induce brain chemistry changes too, and those changes can be very similar to those caused by external chemicals. So, fortunately for me I can feel Good in ways that don't involve pigging out on ice cream, or pounding shots, or smoking meth, or shooting heroin. I can run, I can have sex, I can play video games, whatever it is that makes me feel Good. For some this extends to activities like gambling, or shoplifting, or base jumping, or rock climbing, which have a common thread of risk taking and beating the odds and "winning".
Addiction comes about when our brains are forced to cope with an abnormal balance of chemicals, caused by us regularly altering that balance through external chemicals or through processes. The addicted brain is not accustomed anymore to a normal state, where the balance isn't constantly being thrown off, it is accustomed to an abnormal state and has done it's best to rebalance out of that abnormal state. Here is where tolerance grows, why we need more and more of a chemical or activity to alter the balance in the direction we want, and why many addicts say they no longer really enjoy their drug of choice, they use it simply to feel "normal" even if they have come to hate it because of all the negative consequences. And why they continue despite the increasing negative consequences.
When addicts shut off the drugs or stop the processes, their chemical balance is thrown off wildly in the opposite direction. Alcohol addicts start twitching and shaking, coke and meth addicts crash and sleep for a week, gambling addicts get depressed, all because their brains have become accustomed to a chemically-altered (neurotransmitter chemistry) state. It can take a long time to feel "normal" again as they move through acute and post-acute withdrawl, and meanwhile they have to deal with triggers and cravings that try to drive them back to their drug of addiction.
I think this is important. All this stuff is going on inside the brain, it's all fundamentally biochemical, and process addictions aren't really very different from chemical addictions at the level of what's going on inside. There is some sort of grand unified theory of addiction that is something like the above very simplified cartoon picture, and it includes drugs and it includes processes.
Copyright, 2012, JeffK
The way I look at it is something like this. If I have the capacity to feel Good, say, then what that means is that when I feel Good, certain chemistry is going on in my brain particularly in nerve synapse regions. Natural organic chemicals (neurotransmitters) are being produced in one place, modified somewhere else, absorbed in another place, and there is some mix that at a biochemical level is what is responsible for me feeling Good.