Park tourism is correlated with better mental health, with high economic value, but some aspects of causal mechanisms have remained unresolved. I interviewed 238 outdoor nature and adventure tourists of all ages and backgrounds, at subtropical national parks in Queensland, Australia. I asked whether they visit parks because they are happy, or they are happy because they visit parks. A large majority, 82%, picked the latter. Of those asked, 87.5% reported short-term emotional benefits; 60%, medium-term recovery from stress; and 20%, long-term changes in worldview. Research priorities include: economic scale; experiential components and commercial design for different market segments; and tests of mental health benefits for tourism more generally.