The idea of whether rats with exciting lives would indeed be less likely to become addicted to cocaine / morphine is quite controversial, and there have been so

The idea of whether rats with exciting lives would indeed be less likely to become addicted to cocaine / morphine is quite controversial, and there have been some failures to replicate that original study (references here: 1. https://www.ncbi.nlm.nih.gov/pubmed/9148292?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/9148292?dopt=Abstract and 2. https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract https://www.ncbi.nlm.nih.gov/pubmed/2616610?dopt=Abstract). I think the argument is that social isolation can and probably does make animals and humans more prone to drug addiction, but it's definitely not the only factor, and both animals and humans with good lives can still become hooked.