Su Qing has been dead for half a century. He doesn't remember why he died. He only knows that the water in Qingming that year was very cold, so cold that he forgot everything about his life.
He had lived in that house for half a century, and he was almost getting lonely, until Zhong Linyu appeared.
Can ghosts be emotional? He didn't know, he only knew that he wanted to be with the only person who had brought him warmth for half a century.
Why? In the end, even Zhong Linyu's appearance was just a trap set up by the person who had hurt him.