If life is short, we should expect its shortness to take us by surprise. And that is just what tends to happen. You take things for granted, and then they're gone. You think you can always write that book, or climb that mountain, or whatever, and then you realize the window has closed. The saddest windows close when other people die. Their lives are short too.
Thanks for posting this quote. It prompted me to google Paul Graham and found quite a few very interesting essays that he had written. Thanks for the introduction to Paul Graham