Addiction is a manifestation of brain plasticity where the brain upregulates receptors in response to drug administration.

“addiction is all about brain plasticity. put a certain drug in your system and what your brain does is it upregulates the receptors for that drug which is its way of saying oh I didn't know the world consisted of this stuff good I'm going to prepare for this now and I expect more of it so then you give it more and this is great I'm going to upregulate the receptors again and it comes to expect that this is in the world and then if you stop you have these awful drug withdrawal symptoms precisely because you've changed your system now it's expecting the world to have that.”