Recently there have been articles relating good health status to contact with nature and animals. Most of us would accept these assertions on faith and could give several explanations for the beneficial relationship. The one point I would emphasize is how an appreciation of nature requires physical activity to get out and enjoy it. That gives a simple explanation but I am sure it goes so much further.
Human health and nature conservation -- Burls and Caan 331 (7527): 1221 -- BMJ: "Human health and nature conservation "