Poor mental health costs about a tenth of global GNP. For many people, nature exposure improves mental health. This generates a newly recognised health service value for national parks worldwide, estimated at trillions of dollars annually. It creates a correspondingly large potential new market for commercial nature tourism, as a provider of nature therapies, especially in national parks. Before this opportunity can be realised, however, we must establish whether voluntary park visits generate mental health gains for everyone. Currently, psychology researchers and protected area managers hold opposing views. I therefore interviewed 238 outdoor nature and adventure tourists of all ages and backgrounds, at subtropical national parks in Queensland, Australia, to determine 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. Each timescale corresponds to a different opportunity for commercial nature tourism. Future research priorities include: economic scale; experiential components and commercial design for different market segments; comparisons of individual benefits for people with different personalities; and tests of mental health benefits for other types of tourism.