First, there is a sense of perceived cultural and racial superiority stemming from traditional Korean cosmology. In that great Sinocentric chain of being, Japan was an altogether inferior entity. In it, China was at the top with Korea occupying a middle region, above the "barbarian" peoples among which Japan was also numbered.
So the reversal of fortune, so to speak, where Japan forged ahead in recent history hurts Koreans far more than would have been the case if--say--China had colonized Korea instead. The latter would have been rationalized as merely the "natural order of things," to be only slightly hyperbolic.
Now the point about China brings me to my second reason. Historically, China has inflicted far more atrocities toward Korea than China. Further, the Chinese atrocities were more proximate: see Mao's intervention in the Korean War. So "suffering" alone cannot be the cause of the South Korea's animus toward Japan or South Korea's greater animus toward Japan than China.
My point? Korea is belligerent toward Japan also partly because Japan--unlike, say, China--is not a serious military threat to South Korea for a variety of reasons. Compare South Korea's near-lunatic response to Japan's Dok Do claims to South Korea's more measured and almost docile response to China's claims on Koguryo. South Korea in other words is acting as a bully.]Abiola Lapite