There is always a person who has been dead for thirty thousand years between Wuji and Shangxie.
What is obsession? In Wuji's opinion, the deepest obsession in the world is to spend 30,000 years without any regrets, waiting for someone who will never come back;
In Shangxie's view, the person who had been abandoned by the creatures in the three realms was the stupidest, most idiotic, and most persistent person in the world.
It wasn't until the mystery of a hundred thousand years ago was gradually revealed that Wuji realized that it was impossible to fall in love with someone else again after falling in love with a woman as bright as a star.
What he owed her would take eternity and eternity to pay, so a mere 30,000 years would mean nothing.