"You should say your farewells ahead of time, because when the time really comes…you won't have time to say your farewells."
Somehow I love it a lot that the script went back to this line. It's like an echo that resonates. Even though he loves Song Yi, and not the girl who looks like her, that girl from a long time ago does play a part in his 400 years on Earth. Him remembering his experience and lessons from the girl has a much nicer significance than if the girl was just a fleeting crush-of-sorts.
I teared a lot from this episode because we are finally nearing this point, the point when he leaves. It's tomorrow in the show, and quite literally, tomorrow when the episode releases. [or today, not sure when I'm gonna post this post exactly]