Song Jie's mindset at the time was that he was wholly dedicated to his career, and when he reached the appropriate age, he simply wanted a wife whom his parents would approve of. He never expected to meet someone he desired.
But when he saw Gu Huanhuan, he realized there are people you see for the first time and instantly know they are the right one.
However, upon investigating further, he found out that Gu Huanhuan was already married with children.
At that moment, he felt deeply despondent for a long time. It was hard to find someone he liked, but she already belonged to someone else, so he had to bury his feelings deep in his heart.
Moreover, he was already engaged himself, carrying responsibilities of his own, and he could not let down a good girl.
Because of this, he became depressed for a while. In the end, he decided to silently protect her, which became a belief of his own.