I'm not sure but I'd say because of Japan, Japan more or less did not get involved in many wars as far as I know. I know that Japan had no reason to go against Korea but still did and sided with China/North Korea. Because of that a lot of hate came from that and since then even sometimes now South Koreans are not very fond of Japanese. Also I hope no one takes offense that's what I know I'm not saying all Koreans hate Japanese just some... Hope this helps
