It's part of our evolution. Our ancestors weren't cooped up all the time in caves, they were out in the forests & plains. I'll bet our need for the outdoors is hardwired in our brains. Which has interesting implications for present day people. Is all this time we spend anymore watching TV or ( gasp! ) at our computers partially responsible for the aggression & antisocial/anticivil behaviors that seem to be on the rise in our society? Are we experiencing more mental illness partially because of this? Does drug use in part stem from a need for stimulation that might be better served by contact w/ the more natural world? Would natural contacts better help the supposed rise in ADD than drugs by answering a possibly hardwired need in our brains to expend energy that our ancestors would have used to survive in their world? I'm probably reaching here & speculating too much, but I think these are interesting ideas to contemplate.