From CNN.com:

Why does it always seem to take something like this to move us, however briefly, toward civility and mutual understanding?

Why is it usually in the worst of times that we step back, lower our voices and look for our common humanity?

The carnage in Arizona has stopped America in its tracks. We who disagree on politics and policy are not each other's enemies. We should have known that all along. Yet for whatever reason, it takes the darkest national moments for us to share a collective deep breath and question the way we have been talking to each other and treating each other.

Think back to September 11, 2001-- to one moment in particular. Maybe you've forgotten it.

Why does it take tragedy to unify us?