What, exactly, is addictive about exercise?

I’ve decided that I’m done procrastinating, that I’m actually going to get back into shape. I’ve found in the past that it takes me about 6–8 weeks of pain and reluctance to reach a point where I seem to become addicted to exercise: I feel good afterward, I want to do it, I feel bad if I don’t.

If it’s endorphins, why would it take so long to develop? Is this an actual addiction or is it a mental psych-out? Can anyone find anything besides anecdotal evidence of exercise addition?