It was precisely because he didn't particularly like or care for Song Huanyan that when Song Xiaoyu flirted and seduced him... he took the bait.
But... upon knowing that Song Huanyan and Yun Jianchen got together, all his feelings seemed to have changed.
He was very angry. Angry that Song Huanyan and Yun Jianchen were engaged.
Every time he saw Song Huanyan and Yun Jianchen together, he found it particularly irritating and annoying...
Clearly, the man standing by Song Huanyan's side should have been him.
Maybe... it's only after losing her that he realized how much he treasured her.
His previous feelings for Song Huanyan were rather mundane, just a general sense of fondness... but it was only after he lost Song Huanyan that he found out how much he actually cared about her.