I have no idea why they happen but suspect that it has something to do with the fact that consumption of our doc was an everyday behavior over many years that altered our brain chemistry. Perhaps our brains are just working the stuff out....
I know one thing, i have these dreams every so often and they're quite real. It's a bizarre experience to awaken feeling panicked and unsettled because you relapsed, only to realize after a few seconds that it was only a dream. That moment of realization and relief almost makes it worth it. Almost. When i have using dreams i'm usually off kilter for the first 20 minutes or so of my day.