If the korean government charged tariffs on japanese automobiles and in turn the japanese government put tariffs on korean automobiles, japan's major rationale for imposing these tariffs would be to

retaliate for Korea's tariffs.

This is an act of retaliation given that the Japanese have imposed tariffs in reaction to Korea's imposition of the same. This is aimed at ensuring that there is a balance in trade as failure by Japan to impose tariffs may affect its balance of payments