Running becomes a positive addiction due to the epinephrine hormone the body releases as a response to the natural stress of the activity itself.
There are those who still argue whether running is "natural" or not but I know one thing for sure – running can really get you "high" especially if you push over the 5K limit.
That happens when you run long distances because like in all cases of physical stress, your body releases the epinephrine hormone so that you can cope with the "threat" of a higher-than-usual level of effort.
Epinephrine, coming from Greek "epi + nephros" ("to the kidney") is also known by its Latin name, adrenaline (ad + renes, or, "on the kidney").
Secreted by the glands sitting on top of our kidneys, epinephrine is what all "extreme sports" fans (bungee jumping, boxing, parachute jumping, skiing, car racing, etc.) are hooked on.
Police officers, firefighters, bail bondsmen, detectives, and soldiers in combat are also intimately familiar with the heart-racing, blood-pressure hiking effects of epinephrine. It's an evolutionary solution to the instant need to get the body ready for a fight-or-flight confrontation in the extreme cases.
Running does not present such a do-or-die challenge (thanks goodness!) However the sheer exertion involved in the activity still triggers good amounts of epinephrine get released into the blood stream.