Time...
I notice the older we get, the shorter the passage of time becomes. When I was a bad boy at the age of five, a 5 minute detention in the corner seemed like an hour... Now 5 minutes fly by and I barely even notice. I think this has to do with the aspect that time is relative to our life in our minds. The longer our lives become, the shorter a defined amount of time seems. So make the most of it while you can I suppose.