I've read numerous articles about Mycobacterium vaccae and serotonin. Several are very scientific and technical. This Permaculture article gives more information about Mycobacterium Vaccae and serotonin. It is a much more interesting, relaxing read.
Personally, my favorite toy as a child was dirt...lol. When I worked solely in an office I wasn't very happy. Now, I work in the dirt outside and am much happier. Many gardener friends have said their garden is their "Happy Place".