Why Running is Addicting

I've been sitting all afternoon and my quads ache from not moving. This is what happens after a day or two of not running. I hadn't realized I had gotten back into a sufficiently regular running schedule until last week when I experienced this feeling for the first time in a while. There is this urge to use them in order to relieve the aching, I literally want to go up and down the stairs of this building over and over. And if I'm not stuck here grading lab reports too late, maybe I will go for a real run later this evening.