Your paleolithic brain doesn’t want to waste precious mammoth calories unless you’re actually trying to get away from a cave bear.

I’m like this. Exercise is easy when I’m under duress. I hardly even notice the effort. However, it really doesn’t want to do anything when I’m comfortable. Hate to say this, but I think our first world is way too comfortable. I suspect many of us would be much happier with a slightly harder life.

EDIT: Wasn’t there a post recently about a guy who was trying to rig up some thing to simulate a sabre tooth tiger chasing him for his without.