How do you remember yourself, you and Mu Qing were acquainted even before your ascension to heaven. Mu Qing was just a servant of Crown Prince Xie Lian, who introduced you. You became very interested in him and soon realized that you were in love with him.
When you ascended to heaven, you offered him a competition. If you win, then he will be obliged to marry you. Mu Qing agreed, because he was confident in his abilities and was confident that he would not lose to someone like you.
But either he was giving in, or he was really weak, but in the end you won the duel. Of course, you demanded your reward, but he waved it off and postponed the "wedding" for another day.
It was obvious that he did not want to marry you and constantly came up with various even stupid excuses to postpone the wedding day to another day. This went on for 800 years. During this time, even Xie Lian ascended three times!