I thought this was a very interesting and inspiring article on why we love to run - it seems we evolved to do it, and that it brings back our child-like joy.
A couple of excerpts:
Running brings us joy. Watch small children when they are excited, at play, and mostly they can't stop running. Back and forth, up and down, in little, pointless circles. I remember, even as an older child, I'd often break into a run when walking along the street, for no reason.
Racing along out on the trails, or even through the busy streets of a city, splashing through puddles, letting the rain drench us, the wind ruffle us, we begin to sense a faint recollection of that childish joy. Somewhere a primal essence stirs deep within us; this being born not to sit at a desk or read newspapers and drink coffee, but to live a wilder existence. As we run, the layers of responsibility and identity we have gathered in our lives, the father, mother, lawyer, teacher, Manchester United-supporter labels, all fall away, leaving us with the raw human being underneath.
I think that really resonates with me - especially the part about all the layers of responsibility and labels falling away.