So, addiction is caused by the brain trying to adjust to the unusual level of stimulation (either too much or not enough) by up- or down-regulating the number of receptors on the neurons that process brain chemicals.
It's as if the brain is saying, "So, I guess too much stimulation is the new normal around here," and it decreases the number of receptors to keep the brain from being over stimulated. And when there's a change in that "new normal," the brain has to make new receptors because it's not getting enough stimulation. "Make up your mind!" it says to the person making bad choices.
People whose brains readily adjust in that way to drugs are more likely to become addicted. It's why when I eat even a little bit of chocolate, I find myself craving it for hours afterward.
I think that people who are hyperalert naturally produce more chemicals that stimulate the brain, producing a higher degree of alertness. But the delivery of stimulating chemicals is neither constant nor consistent, so it sometimes drops off. When that happens, the person gets depressed because the brain has responded to the hyperalertness by down regulating receptors. Not only does it drop off sometimes, but sometimes it becomes too great, to where there are too many receptors and the brain is overstimulated. One of the results of that condition is that the person becomes angry or combative, I think.
So, too much or too little stimulation produces a fight or flight scenario, with depression being one version of flight (playing dead?).
The cure for all these states of being is time, really, though sufferers are usually in so much pain (or euphoria) they don't want to wait for the brain to adjust to the change in chemistry. So they do something--often something foolish or unhealthy or costly or risky--that works, but only for the short term. And the associated costs are usually high.
The alternative? Recognize the impulse for what it is. Wait, or if you can't wait, do something that will work but will not cause harm.