IMO, the Rat Park experiment has absolutely no relevance to humans, however intuitive it may seem. This is because rats are still driven by the survival instinct and need to be able to fight, fuck and feed in order to achieve this and being addicted to drugs is not exactly conducive to survival in the wild. However, humans no longer require these instincts. With the coming of civilisation, all our basic physical needs are fulfilled, leaving us free to explore the desires of the mind. Paradoxically, this means that our needs will never be fulfilled because our imaginations can always create more. Therefore, its all about fulfillment. It is easy to fulfill a rat's needs with Rat Park. But it is impossible to fulfill a human's because we will always want more than we can have, and never find true happiness.
TLDR, Rats aren't people...