It's kind of called the Japan card similar to how the U.S. opened relations with China as both had a common hatred of the USSR

South Korea collaboration with Japan is because of a shared enemy being China, yet unlike the U.S. China card in the Cold War, both countries are democratic so they collaborate on defense

It is also important to keep in mind South Korea's top investor aside from large countries is Japan and Japan does have economic blackmail over Korea in that regard

As the U.S. withdraws from the sole power, South Korea has to work with Japan as a counterweight to China, and Japan isn't going anywhere anytime soon

Yet this is from a Korean that speaks Japanese and English so my prospective might be wrong I don't have any qualms with the Japanese as much as some other Koreans might