I think that the overall compelling things about Dong Kyung and Myul Mang's relationship is that its like a tragic circle. One half has had so much of their life go wrong through no fault of their own, while the other causes disaster just by existing and cannot prevent it. They've both been powerless to how the world works. One of them is dying while the other was never alive in the first place. Again, they'd both just accepted that that is how things are. They are two sides of the same coin and found each other by a twist of fate and from knowing and loving the other they both realised they want to live. But they can't without at least one of them dying in a couple of months. They match each other so well and it is just a Tragedy.
The more I think on them the more I realise how well their characters contrast and compliment each other; she never tells the truth on how she feels and he always knows how everyone feels but put in the effort to learn how she acts and thinks because he noticed that she lies. He's never been wished happy birthday before, she remembered this and made sure she celebrated with him. They just Know each other; the gift of seeing her parents, the single second;They just work so well together and I have emotions and need to rant them before i forget them so voila have my thoughts