Why do people disagree?

It seems a stupid question. Maybe it is. Or maybe not. Let us assume, for the sake of argument, that we are all rational beings. We possess average intelligence, adequate and more or less similar education, and copious exposure to all the information the information age generates in such profusion. Let us suppose further (granted, real life isn't always like this) that we all employ our educated and informed rational faculty in good faith. We have no vested interests to defend. We are open-minded, ever willing to acknowledge ourselves wrong if shown to be so, or to change our opinion if a more persuasive one comes along.

Is there any public issue about which well-intentioned, intelligent people do not irreconcilably, vehemently disagree? Take Japan's most pressing problems — the struggling economy; the declining and aging population; increasingly strong and assertive neighbors; radioactivity. On none of these issues is there anything close to a consensus, either among experts or laypeople. On the economy, there are advocates of government spending and advocates of austerity. On population decline, some insist mass immigration would revive Japan while others maintain it would be Japan's ruin. On menacing neighbors, Japan should revise its antiwar Constitution, strengthen its armed forces and refuse to back down; or no, it should do the exact opposite: apologize anew for wartime sins, preserve the Constitution as it is and rely on soft rather than hard power. On radiation, scrap nuclear reactors, their danger having been proven beyond a doubt; or restart them for the sake of the economy, the damage of even the dreadful Fukushima No. 1 meltdowns having after all caused no immediate deaths and, so far, no evident health deterioration.