I understand why Japan wants nukes. But I don't see how the US would want Japan to get nukes. A nuclear Japan would increase China's power relative to the US enormously. A slight increase in Japanese peace of mind results in a tremendous decrease in American regional influence.
Because it could prevent both a war and a human tragedy. Our nukes probably won't always be a deterent to North Korea attacking Japan and we shouldn't have to put ourselves in the position of risking WW3 to defend a country with the means to defend itself.