The first reason that it pisses me off is because there should be no excuse for wasting your life. Sitting back and letting life happen to you, or worse, helping it drag you down, is not what you were made for. You were made for more than drinking, partying, and sleeping around.
The second reason that YOLO pisses me off is our soldiers.
How dare you throw that phrase around, when our boys go into battle, painfully aware that you do indeed only live once? Do you think that they're thinking they have a second chance as bullets rip into their flesh, as the heat from an IDE sears their face, as shrapnel punctures their lungs?
How dare you live in the freedom those men and women bought by laying down their lives only so you could waste yours? YOLO! Yes. Now make something real of it and stop insulting the men and women of the military and anyone else who has laid down their life so you might have your precious freedom.