I believe that you’re born with an unknown, but predetermined and finite number of heartbeats. Once you’ve used them all up that’s it – your time has come and you die.
The result of this is that anything you do which increases your heart-rate (exercise, taking the stairs when there’s a perfectly good elevator right there, leaving the house, generally walking around, etc) is just using your heartbeats up faster and shortening your life.
Update!
I have learned that I’m far from alone in thinking this! Check out Heartbeat Hypothesis on wikipedia. Somebody who clearly has no respect for science has flagged the article as “dubious,” which I take some issue with. The article also says that fit people live longer because they have a lower resting heart rate than unhealthy people, which is an interesting interpretation but is entirely missing the point, I think.
How does one edit wikipedia articles?