She and Su Yu really didn't have any love at first sight.
She wasn't someone who easily got emotional, and it has to be said, Su Yu's way of setting traps for her was indeed so deep that... she eventually got caught in it.
Without the later days spent together, Su Yu's feelings towards Han Jing might have only been a sense of duty, without any romantic feelings between them, nor the many changes in life perspectives that came later. And Han Jing's feelings towards Su Yu might only have been a few visual delights and an inexplicable warmth when he occasionally treated her well, but these don't constitute love.
So it really was, over time, as they started to sort out their feelings, they found that they truly loved each other, and each had already become the most important part of the other's life.
