I'm so humbled by this honor

Why do people say they are "humbled" when they receive some kind of honor? Is it because they don't want people to know that their inner dialog is all "check me out!"? Or is it because they doubt their ability to live up to the honor? And if the honor is a new job (Supreme Court appointment, for example), does this instill faith in the constituents? What message are they trying to send?

I guess I've always found that word choice to be a little weird. "I'm honored" would be cool with me.