This one, he has to think about. He has many to choose from. But many of the memories were tinged with pain as well, because on the boat while they ate lotus seeds he was suffering. And when he'd brought home Emperor's Smile for him, they were hidden away for Wei Ying's safety. He was prisoner.

And so, he thinks, it must be an earlier memory. Before they graduated, even. Those memories are untainted.

"The promise we made." It was when he realized Wei Ying is just like him.

It was when he realized, Wei Ying is someone worth his affections.