This, my friends, is
science.
20% of Vietnam War soldiers were addicted to heroin, 95% stopped using when they came home. If "a more pleasant cage" was the reason for people to stop using, then 100% would have stopped using -- because no matter how crappy your life in the US may be, it's still a whole lot better than being in the swamps of Vietnam.
Also -- the "sad" and "lonely" rats chose the heroin bottle while the "happy" rats didn't. Did anyone consider WHY some rats were sad and lonely to start with? Was there maybe a genetic predisposition to be sad/lonely/prone to addiction/aggressive/happy/etc?
Did you notice that the research quoted is also largely over 30 years old?
I follow quite a bit of current neuroscience research, and what seems to be coming out of that is that just as some people are born with a latent
physiological tendency for getting eating disorders, depression, or schizophrenia, addiction is probably something similar. YES, a
disease. Imagine.
There's research that seems to point to some mental disorders having connections with autoimmune disorders, but I haven't seen any clear research that explains how yet.
Also, as Seren points out, there are innumerable examples of people who have perfectly good lives and still become addicts.