The clueless Song Weiming became the "excuse" for their dates.
Perhaps Ge Tianhe didn't think much about it either, just wanting to be closer to his childhood sweetheart. Maybe over time, she would notice his merits and fall for him. But unfortunately, he didn't realize that there is a kind of distance in the world where being too close causes you to overlook the people around you.
His childhood sweetheart didn't fall for him; instead, she fell even deeper in love with his "friend" he mentioned, Song Weiming.
Clearly, there was no substantial interaction, yet his childhood sweetheart loved Song Weiming so deeply, perhaps because she was not in love with Song Weiming but with the imagined version of him.
This led to a problem. When she discovered Song Weiming had a girlfriend and was preparing to get engaged, she was greatly shocked and nearly lost her mind.
"Why doesn't he love me?"
"No! No! No! He obviously loves me, so why has his heart changed?"
