Lately there has been a lot of attention brought to the fact that soil contains enzymes that can cause you to feel happier. I always had a hunch that this was the case! Here are some excellent articles about how and why gardening makes you feel better:
Can gardening cure depression?
Antidepressant Microbes In Soil: How Dirt Makes You Happy
Experts confirm how gardening can cure depression
Is Dirt the New Prozac?