The Kim's family business was sabotaged, the heiress of Kang's were quite dedicated to their empire. 

Jongin was blamed for not taking a business related course, Seulgi was worried about his father's health condition.

The Kims turned to the Kangs for help, but the head of the family was sick so Seulgi's decision was on the line. 

To make the long description short,

It all leads to marriage.