Way Too Good Customer Service
Unlike in the United States, where restaurant employees are pretty compensated and may live comfortably off their salaries, workers in South Korea are paid well and motivated to provide exceptional customer service while on shift. As a result, they will keep filling your glass until you leave the restaurant at the end of your night out.
You didn’t request the pounding headache you’ll have tomorrow morning even though you may have kept drinking. However, in deference to your server, your glass was kept refilling. In South Korea, it is customary to communicate that you are finished drinking by leaving some of the drink remaining in the glass.