Now, I'm basing this meaning off of just what the lyrics say, I'm not trying to find metaphors or anything. I think that this song is about an abusive relationship. I mean, in the song, he says that his seat next to his girlfriend was taken by a dude who asks her about a scar she has. He then says that he gave it to her months ago, and he's sorry. Sorry to those people who blindly sing along to this song thinking that it's as simple as saying to party while you're young. Pay attention to the lyrics you're singing when you sing them.