Why do images of the second plane attack on 9-11 make me cry?

The only explanation I can come up with is that, while watching these events now, 9 years later, you're not only able to see how devastating that particular moment in time was THEN, but the gravity, the impact that it had following. Every horrible thing that has come about because of it, all rolled into one single event, that one single iconic image 9 years ago. The ugly side of retrospect. The flames, the planes, the horror on all of those faces, and to now know that it was just the beginning. A very depressing thought indeed, and I would say, worthy of a cry, even if that realization wasn't a completely conscious one.