He left because he fell for another.
She left because watching him be with another was killing her.
Literally.
She left with a seed he did not know of.
Putting her affairs in order, she is forced to return.
Once upon a time, a woman running into a cafe, forgetting her phone at the cafe, and suddenly he bumped into a very gentleman, thoughtful, and sweet person who she didnt know that the man would change her life forever.
(You'll know how polite hung it is by seeing all the episodes carefully, you'll find a fact or hung's personality each episode)