Sometimes the bodies own correction strategies become part of the problem. Our brain is wired to relieve stress by seeking things that bring pleasure. In our prefrontal cortex we all have a hierarchy of what the organism seeks for pleasure to counteract stress. When we’re stressed we look to that “list”. In a normal, healthy environment that’s OK because nothing is unusually primary. But a drug totally floods the pleasure pathway and it roars to the top of the list. The more one relies on that drug (or alcohol, sugar, cheap sex, gambling, games, etc.), the more neuronal pathways get wired and the more receptors get created and the more one HAS to have “fix” that to satisfy those receptors. When one doesn’t take the drug/sugar/sex/etc., s/he gets stressed because the receptors aren’t being filled. So one gets trapped into a vicious cycle to relieve the stress.
The really “mean” part of this is that it is the pleasure-seeking that is stimulated, not pleasure itself. Drugs/sugar/sex/etc. have their biggest pleasure response the first time and the actual pleasure fades with repetition while the pleasure-seeking gets stronger. So one needs bigger and bigger hits to get the same level of pleasure—and thus producing even more neuronal connections and receptors for the addiction, so it gets even harder to get out of the addictive habit.
Now the good news is that those neurons pathways and receptors will die out over time if they aren’t satisfied. But one is “craving” the fix while they fade away, which of course creates stress. More good news is that there are effective strategies for calming those cravings.