Poor Yuan Yuan didn't even realize he loved her until she was dead in his arms in the previous life, the technique he learned from the throne that he rejected in this life literally suppressed his emotions and forcibly erased any memory that wasn't about training to become the strongest. He was a cold jerk to Yuan Chu because he literally kept having all of his memories of her not related to training/getting in the way of his training erased so it was almost like he was meeting her for the first time over and over. All the sweet moments they had shared were thrown in the brain's recycle bin by the technique, which lead to him breaking her heart. Yuanchu had her heart sealed with those Golden chains we saw in an earlier chapter to not deal with the heart ache sealing all her memories of loving Yuanyuan. After she died, he overcame all the brainwashing the technique did to him and snapped. He became obsessed with bringing her back to life going so far to use the cursed mirror Yuan Chu has been looking for and putting a piece of his soul into her so she would be reincarnated as well. The cost of using the mirror is what you cherish most, Yuan Yuan lost his memories of the previous timeline as the price.