Motorcycle riders have a common saying and belief: "You won't see many motorcycles parked at the therapist's office." Suggesting that riding a motorcycle may avert mental health problems. Maybe you don't see bikes parked there, either.
I happened upon an interesting article, "Jogging in forest twice as good as trip to gym for mental health." The study seems to indicate that communing with nature - "being around trees and grass" - lowers brain stress levels and soothes the brain that's suffering from depression and anxiety. Working out at the gym is not effective, because "only activities carried out in a natural environment outdoors were found to be associated with a lower risk of poor mental health."
Bicycling isn't mentioned specifically. And there are stress-filled situations, from time to time, for a transportation cyclist. (Although they are much less frequent than for a "transportation motorist." Or so my experience would lead me to believe.) But I can totally identify with the therapeutic effect of bicycling, particularly when the scenery is nice and traffic is light. (ALL of my bicycling is "in a natural environment outdoors.")