one true thing

I saw HP and the Deathly Hollows 2 weeks ago and the thing that struck me most is how true that one line of When the Years Fill In turned out to be. "It was war, you were seventeen, and the only promise anyone had ever made to you was to die on your behalf." There is something about watching that battle that sings with that sentiment. It may very well be the truest thing I've ever written? What is the truest thingb you've ever written?