Time is fleeting. I made this android wallpaper so I would be reminded of that fact. Every time I look at my phone I think "am I doing what I really want to be doing? am I making the best use of my limited time?"
Perhaps a bit morbid, but very realistic. Death comes for us all, even if you ignore it. By consciously forcing yourself to confront it you can reduce the chance that you'll wind up with a ton of regrets when you're old.