Why should I live life to the fullest?

People always say you should live life to the fullest. I justify my laziness by asking myself "why should I". I don't feel sad at the moment so I think i'm pretty happy. When I look back on my life I think, "well I haven't done very much in my lifetime". This bothers me but not enough to change the way I live and go skydiving or something. Our society/culture makes us believe happiness is the sum total of our accomplishments, but there is more than one happiness theory. So should I start "living" more so i'm not depressed in the future lol, or should I just be myself?