For me mountain biking forces me to be in the moment and all stress, worry, anxiety, anger, or anything else melts away into the trail behind me as my focus is on the trail in front of me.  With the amount of focus it takes to rip down technical sections, or any section for that matter, there isn't much room left in your head for any negative thoughts or emotions.  The bike, the trail, and I flow together into a place in my mind where nothing else exists.  When I find myself heading to the trail to find peace rather than enjoyment the relief is only temporary, but I find when I'm done riding, I usually have an entirely different view of whatever the situation was that I was temporarily trying to escape.  And other days when I'm not feeling like an intense ride I take it slow and just go enjoy nature.