That our ancestors made incredibly bad mistakes, but we learned from them. I mean it took a war and the near destruction of the entire human race to get it, but Union, at least, learned not to pull stuff like that.

As for me? That some humans from half a millenium before any of us were born were dumb, and some of them are still dumb, but the only reason I'm alive to say any of this is because of Hiccup, so I'm going to give them the benefit of the doubt.