What brought Shenwei his soul in the end of the book was the fact that Shenwei was willing to give up his love for Kunlun Jun for Kunlun Jun's sake. He finally made the first choice of his entire life and he chose to sacrifice the only thing he ever had so that Kunlun could live.

When Shenwei was willing to erase Zhao Yunlan's memories, he was giving up his love. He had to let go of the only thing he ever wanted, he ever had, and that he ever cared for, and that's what ultimately gained him enough good karma that he was granted a soul.