There's a bug about running that you catch. It could be the exhilaration di of propelling your body through space, or the pounding on the ground that sends sensation up your bones all the way to the pleasure centers in your brain, or it could simply be the sheer satisfaction of having done something good for yourself. Whatever it is, running can be addictive.
The psychologist Mihaly Csikszentmihalyi might describe the experience of running as "flow," the state of mind in which you are fully immersed in what you are doing. Or it could be what William Glasser calls, "positive addiction," where you perform a repetitive activity without self-criticism or judgment that has a beneficial effect on your mind and body.
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