Responses
Sinwoon's biggest mistake is that he started to take Yoonsung for granted. He started to assume that he will always be there. This is not how it works. Both partners must think of the other's feelings in order to make it work and keep the relationship strong. A relationship is something you maintain. There may be ups and down, but as long as the partners keep being on the track, they can be happily together.
I am somebody who is in a relationship for years. Trust me when I say that butterflies can be rekindled. They may disappear at some point, but they can be resurrect if both partners have a good communication. If the chemistry was really good at the start of the relationship, then butterflies can come again. Don't worry and give up.