It's a very complicated issue, but Korea was occupied by Japan for 35 years. It wasn't only during the war. The occupation only ended when Japan was defeated at the end of WW2.

I know this seems like uplifting news, but anyone familiar with S.Korean politics will know this is just a political game that gets played between the right and left.

This is very simplified but:

When the left have the presidency they seek better communication with N.Korea and use Japan as a boogeyman.

When the right have the presidency (which they do now as of this year), they seek better ties with Japan and use N.K as a boogeyman.

It also coincides with N.K acting up with missiles and threats. They were relatively quiet when Moon was in power. But they were Rocket Man city when Park Geun Hye was in power, and they are becoming like that again under Yoon.

There were long lasting NO Japan boycotts under Moon.