Imannuel Kant failed to understand either logic or the fact that values are multidimensional and hierarchical, because he failed to realize that you can avoid hurting people by merely constraining your actions and desires to nonaggression, and then desiring whatever. He failed to understand that intentions toward ends that are not simply human happiness do not necessarily conflict with that human happiness, and to believe that the knowledge burden of knowing how to create human happiness falls upon every man at all times is, well, an interesting thought experiment but not a sufficient general solution to the problem of finding an objective morality.
Discussion
And, hinted at in here, it is extremely arrogant to assume that we necessarily have that knowledge for how to author happiness, in a way more effective towards that end, than if we forget about trying to be happy and just do what humans do within the constraints of a different philosophical foundation than the one he put forth.