I've read (oh boy, how I've read!) that runners can get addicted to running. There is apparently a "runner's high" from the endorphins that are generated when you run. I think that endorphins are the body's natural painkillers, so this is an all natural version of one of the cheaper addictions out there.
What I've noticed is that running for 6 or 7km no longer does it for me, if indeed it ever did. In order to come back from a run feeling on a high, it has to be longer. Thursday night's trail run, up from Newlands Forest to Plumpudding Hill and back, taking in views of the city and the Southern suburbs, did work. I pushed myself a bit, despite walking some of the hills, and came back really glowing. It's a combination of the run, the views, the exercise, the cameraderie of running in a group, and maybe some endorphins.
Sometimes I feel like a real runner.