China warred and then subjugated Korea on-and-off for a millennium.
China and Japan warred on-and-off for about two centuries.
Korea was the little bitch third-wheel that was passed between the two countries.
Japan raped and pillaged Korea for a century - literally trying to extinguish Korean culture by teaching Japanese and banning Korean signage.
Japan then raped and pillaged China for a few decades.
So east Asian countries have not historically gotten along.
Economically, Korea and Japan dictated strict foreign tariffs to protect domestic goods, so there's even a brand-alignment that's (un)intentionally created - e.g. circa 2002, a Ford Taurus or Toyota Camry cost the equivalent of $70k in South Korea, where as the Hyundai or Kia equivalent car costs about $25k.
More recently, Korean firms and Chinese firms have literally stolen Japanese intellectual property and processes (1. "We'll manufacturer your goods for you. For cheap!" 2. "Wait, we just learned everything we need to make televisions." 3. "Fuck off, we're making our own TVs." to enter high tech industries to great success.
So international lawsuits ahoy.
East Asia is a silly place. Except silly would be the wrong word considering all the horrible shit that's happened between the three countries. :|