Jung Hee and Gyum Duk (the monk friend) seem to represent two extremes in Buddhist philosophy – sensual self-indulgence and self-mortification, respectively – while Dong Hoon represents the “middle way”, as @maddymappo and @h2ogirl explain.
Maybe there is a parallel between Dong Hoon and his monk friend. They both lock themselves in a room, try to find peace within themselves, and shut off the people around them. DH does insulate himself; his mother said he did everything and worked the hardest but never asked her for anything. DH has never acknowledged his own needs. He works for others. It is as if the desire for worldly things would taint his heart or purity. However, what distinguishes DH from his friend, Gyum Duk, is that he hasn’t literally shut himself off from the material world. While Gyum Duk has committed to a life of asceticism, DH instead tries to stay above materialism, or between it and his principles – the so-called “the middle way”.
Jung Hee, on the other hand, lets it all hang out. She is all unquenchable desire, she is self-indulgence, she is the other extreme to the monk’s self-mortification. She never shut herself off from her emotional attachment to the monk. She cannot break free, so to speak. Jung Hee is admirable though, because she shows that love is not fickle, that it can last even for decades. She is clearly a beautiful woman and could have met men and had children if she set her mind to it, but she just would not or could not let the monk go. In a way, she has locked herself away too. Was that cruel to the monk? Just as cruel as he was to her, I suppose. You can leave the world, but the world doesn’t leave you.
So far, DH has tried very hard to live his life in the middle way, not showing his desire or greed for the world, just living life righteously. But now that he is the crossroads, he is finally opening up to what he wants.
In Zen Buddhism, an individual who seeks enlightenment in pursuit of Nirvana needs to avoid both sensual self-indulgence as well as self-mortification. Since Buddha himself advocated avoiding both these extremes, the path laid down by him came to be known as The Middle Path. By “middle”, Buddha essentially meant that one needs to embrace both spiritualism as well as materialism, just like the front and back sides of a sheet of paper.