Heroin is a short-circuit of the brain's reward system. It's a validation of my point, not a refutation. That reward system exists to make us feel good about things that enhance our survival. That means, throughout history, the vast preponderance of things that make us feel good (eating, exercise, sex, etc) are also evolutionarily beneficial. Sure, lately, we've become very good at targeting the pleasure without the effort required to achieve the benefit, but for things like posture I'd still contend that a comfortable position is likely the one that's best for your body. See: slouchers have less back problems than people who sit straight in their chairs.