Also, rat studies indicate that environment plays a large role in the symptoms we see as addiction - the inability to stop, constantly seeking more of the drug, etc. These symptoms tend to stop when the rats had adequate engagement, weren't overcrowded, etc. Even when they continued to have access to the drug, they tended to stop.

We saw something similar in humans after Vietnam. The soldiers over there were doing any and everything to avoid the horrors of war. Even when they came back with PTSD, we didn't see a huge uptick in drug addiction. This requires a lot more study, but there are some pretty good indications that people get addicted when their lives suck and they don't see any workable options available to change their situation. Addiction may be a disease based in despair more than an innate status in the brain.