South Korea–US relations were a success story of international cooperation during the Cold War on the basis of the robust bilateral alliance. In the post-Cold-War era, the security environment of the Korean Peninsula and the importance of the United States for South Korea's security are no different from that during the Cold War. However, unlike the past, South Korea has not complied with US leadership during the Kim Dae-jung and Roh Moo-hyun administrations, but has adopted policy opposed to that of the United States. This study started with the question of what variables explain South Korean policy toward North Korea in the Kim Dae-jung and Roh Moo-hyun administrations and why South Korea has not followed the US leadership since the Kim Dae-jung presidency, unlike in the past. In order to answer the above puzzle, this study tries to analyze the puzzling situation with the feature of constructivism, such as the change of national identity. From this study, I found that generational change in South Korea from the Korean War generation to the 386 and Internet generations played an important role in the change of South Korean national identity. Also, South Korean younger generations' voting behavior has affected South Korean engagement policy in the Kim Dae-jung and Roh Moo-hyun administrations.