In my experience, runner's high is only obtained after you "hit the wall" and get your "second wind". You have to run past the wall and hit a second wall before you get "runner's high". I always thought it was a result of the release of endorphins.
In order to reach a runner's high faster, you have to run at a high level of intensity for an extended period of time. The more you train, the better level of conditioning you reach, which means you have to continue to push yourself even harder to reach a real runner's high.
Evolutionarily, I think it's a survival mechanism. It allows you to push past all feedback from your body that tells you to stop, and continue anyway.