All sorts of rewards are built into our marvelous organic human computer, mind/body joining with our surroundings to produce wonder.  Evolution gave us a reward when we smell a flower or eat a peach.  And even when we get angry, anger/fear  is part of the fight or flight response, so integral to survival.

With all the over stimulation of our modern age, the reception becomes cloudy.  Dopamine, the brain chemical that transports the reward stimulation runs low after a day of drinking high fructose pop and eating grease, salt, and sugar and playing violent video games.  Not to mention injesting speed, cocaine, caffiene, booze, ritalin, and nicotine.

Once the dopamione receptors have become clogged with dopamine, the receptors can't accept anymore stimulation and often there is no dopamine left to do that stimulation.  This is what propells addiction.  Eat one more donut, no stimulation happens, so you eat 2, or 3.

When you exersize, run/walk, bike, ski, or simply do yoga breathing, the dopamine receptors are magically restored.  The organic computer is reset.  It happens for me after about 45 to 90 minutes, I find myself noticing that I lost track of exactly where I am on the trail I am running on.  That's how I know the reset has happened.

This is why/how exersize works, in terms of addiction to quantity, of whatever substance/stimulation one chooses.  Quality returns to experience, you smell the flowers just like you did as a child.  Stress evaporates and you are ready to go again.  Hopefully in a healthier, less addictive positive direction.