Responses
It depends on how you view it. If the end result is not that important to you but instead the ride is maybe you can enjoy it. But for others when the ending matters knowing in advance it won't work and seeing how cute they were gives a bitter feeling.

Why people always make such a big fuss about suji and sumin's past?
The story is so beautiful, cute, lovely, and pure.
When you have the ending, it doesn't mean we don't have look back.
Every story, even us in the real world, have bitter and sweet memories that we want to dig up again.
Wise man said.... don't judge people if you don't know their past.
For WDTFS.... I was regretting for judging baek seju for troubling su-min dan sung-min so much in the main story.
Because in this flashback.... she's so in love with su-min.... and everything mess up just because of one mistake while she's drunk. Now I know why she hates alcohol so much.