It might also have something to do with negative-ions.
Concentration of Negative Ions in Different Environments
Environment Concentration *
Waterfalls 95,000 - 450,000
Mountains, seashores, breezy forest 50,000 - 100,000
Breezy country meadow 5,000 - 50,000
Cities 100 - 2000
Rooms and offices 40 - 100
Rooms with Air Conditioner 0 - 20
*Approximate Concentration of Negative Ions per cubic centimeter (cc3)
I also read in another article that pine/evergreen forests were especially beneficial. Not sure if any of this is true, but I prefer to spend as much time as possible in the forest. It helps that my back yard is up against a national forest. My wife and her friends think I am crazy. Isn't it boring to spend so much time by yourself in the forest - they ask before going back to the tv?