This is because Koreans are educated that 'Korean people in general dislike Japan because of the past' but a korean individual do not have any grudge for their past invaders because they are not the one who suffered Japanese occupation.
Now, I am a korean and I don't have anything against Japan. I have many Japanese friends and I'm very interested in Japan and Japanese culture. but if I was asked how koreans feel about Japanese culture, I would say the koreans are not very fond of Japanese because of the history.
I'm not generalising that ALL KOREANS are neutral about Japan. The koreans as far as I know have never expressed any anger for their neighbor. They just 'know' that koreans dislike Japan but themselves never have any anger for the nation, including myself. So, Don't take literally when you heard a korean person saying 'Koreans and Japanese don't get on very well'
Peace