Relations between Koreans and Japanese have never really been good. Going right back to medieval times, Wokou pirates from the Japanese Islands regularly attacked the southern coastal regions of the Korean peninsula and while the Koreans did retaliate with successful attacks on Japanese soil, the problem only resolved itself when the Koreans granted certain trading privileges to the Japanese and with the growth of a strong central authority in Japan in the late 1500s and early 1600s.
By the late 19th century, Korea was the focal point of power struggles between China, Japan and Tsarist Russia. Japanese machinations eventually led to the annexation of Korea in 1910. The occupation was brought to an end in 1945 (when the Korean peninsula was partitioned) but resentment about Japanese crimes and atrocities remain to this day in both North and South Korea. Meanwhile, 63% of Japanese people think that Korean demands for an apology are "incomprehensible".
There is a long history of anti-Korean sentiment in Japan. The Kanto area of Japan (which included Tokyo) was struck by a massive earthquake in September 1923. Subsequently, rumours were spread by the government of the time that members of the Korean community were poisoning wells and planting bombs. This led to a bloody massacre of Koreans in which some 6,000 innocent people were murdered.
These days, there is also openly expressed resentment at the success of Korean pop groups and soap operas on Japanese TV. This resentment has at times led to demonstrations outside TV channels broadcasting items of Korean origin calling for an end to the "Korean Wave" and for TV stations to get rid of the "cockroaches" a term that far-right Japanese use to refer to Koreans.
There are just under one million people of Korean descent in Japan. Most of these are Zainichi Koreans, descendants of those who moved (or were forcibly brought) to Japan in the 1910-45 period. They have tended to face discrimination and racism, and more recently open hostility from far-right groups. There have been noisy demonstrations outside Korean schools with some of the more extreme groups shouting about Koreans having "filthy blood" and calling for them to be massacred.
All in all, the Japanese have not been very good about acknowledging crimes committed by their country in the first half of the 20th century. Hence, the extreme nationalism that has (for example, in Germany) been challenged and discredited elsewhere, is far more part of the mainstream in Japan.
There is also the painful reality of Japan's economic decline. A generation ago, South Korea was far poorer than Japan. Now, there's no real difference in the standard of living in the two countries and, what with Japan's anaemic growth rates, it's likely that South Korea will be the richer country by the end of this decade. That will be a hard fact for many Japanese to swallow.