I hear it everyday at my school even my computer teacher says it. When she said it once I raised my hand and said "I'm putting my headphones in without question" and she said "What for" and I said "So I don't have to hear you say that forbidden word" and the whole class laughed then I get sent to the office. I seriously hate that word.
Drake has a song called the motto and he says it in the chorus so everyone trended it and put it as a hash tag yolo and troll are two very annoying word in my opinion along with other dumb acronyms and words
Well technically it's not a word... But anyways, people started using it because it's SUPPOSED to me saying that you only live once, so you should make the most of your life and live it to the fullest. But as it caught on, people started using it as a way of an excuse to do stupid things.
Some people just say it to be annoying. Also, kids like to sometimes make up different meanings for words like that. For instance, I know kids that joke around saying YOLO me YOU OBVIOUSLY LOVE ONE DIRECTION It's a trend. It'll go away soon. Trust me.