"YOLO" stands for You Only Live Once. You do, in fact, only live once. It's still a startlingly stupid thing to say.
The reasoning behind this saying is precisely backwards. It was popularized in a hip-hop song by Drake, and has since become the viral de facto motto for anyone who wants to justify moronic behavior.
Consider the Bond film, You Only Live Twice. Fleming titled his book that because James Bond does take risks, does enter into danger, does act as though there are no consequences. Bond, in other words, acts as though it isn't the case that you only live once. He does things that you shouldn't do if you only live once.
So why do dumb teenagers everywhere think it's a good thing to yell when they do something reckless? Probably mostly just because they are dumb teenagers.
This isn't just grumpiness. The fact that we only live once is a good reason to make good use of our time, to weigh the consequences of what we do, to make the most of it. It seems like a good reason to be careful, not careless.