Why are more and more children turning into hooligans? What are we doing wrong?

I was on my home from shopping on Saturday and I got on a train. When it stopped at a station, I heard a thud and realised that someone had thrown something at my window. As I looked out, I saw a girl in a track suit, no more than 13-14 who then went on to stick her fingers up at me, with no provocation or anything of the sort. I was angry at the time but now I feel sorry for her. But I don't understand where this mentality to harass others comes from! Any thoughts on this?