I've seen comments all over the place wondering why Choi Young won't go with Eun Soo through the portal. The logic goes (and it does make sense) - separation will turn them into the living dead, Eun Soo can't stay in the past or she will die, so why doesn't CY just tell her "Imja, I am going to the heaven world with you." Otherwise his options are to have her die in front of his eyes (her choice) or shove her through the portal and break both their hearts (his choice).
But it makes sense to me - CY is pathologically duty-bound and almost as pathologically unused to putting his happiness first and being selfish. And for him to follow Eun Soo to heavenly world, to live in bliss (precisely since it will be to live in bliss) is something I am not sure he will allow himself. Sure, Eun Soo comes first to him, before King and Country, but King and Country are very important nevertheless. I am sure he wouldn't think twice to go through the portal if it would mean saving her life (he ditched his post when it meant saving her life - remember, when Yuan threatened). But he doesn't have to go through the portal to save her life - she can go by herself and be equally saved. So I don't think it would even occur to the poor guy to think of going. If Eun Soo asked, I think it would be different, but I can see why she doesn't - she thinks he is happiest in his world.
Oh, you Gift of the Magi darlings.