"We fight for what we left behind" means we never let go or forget the people or things in the past. In other words if we love someone and they die or just move somewhere else, those you loved never truly leave us. There are things that death and time cannot touch. The world needs someone they can admire from a distance; from a very far distance. And the people you love will always be in your heart and you never want to forget them so you keep holding on tightly to the memories of them, which means for fighting to keep those memories alive in your mind.