NA/AA is bogus

[JuliaL]

Hence: disease.

No... This way the brain is diseased by default.

Neural behavior is electro-chemical. The brain is a neural net whose overall operation we experience as consciousness. Within that framework, desires are equilibrium conditions arising from chemical activity within and among the neurons. Many classically addictive substances disrupt this chemistry in neural pathways interfering with what was generally adaptive behavior.
Morphine/Heroin/opiates in general mimic endorphins in normal pain suppression pathways. They thereby directly satisfy the desire to be pain free. Sometimes this is maladaptive.
Cocaine potentiates dopamine dependent pathways by increasing dopamine concentration. Exactly what that means on a neuron by neuron basis is unknown but people are working on it. The short term net result of having your pleasure circuits jammed full on is pretty good: euphoria. But it also hijacks your will and crowbars in a desire to take more. The exact mechanism is unknown but being researched. The general idea is that the drug causes persistent changes to the network which do not immediately (if ever) return to the status quo ante when the user stops using. In fact, trying to return to pre-usage conditions can be very unpleasant.

So there is truth to the statement that 'the brain just wants to have fun.' But that desire can be quite self destructive and if not actually a disease, the for sure it's an inclination toward disease.