Unless my memory fails me, the next time Lan Zhan saw Wei Wuxian after turtle cave was when Wei Wuxian returned from Wen Chao dumping him in the burial mounds? It's at least one of the next times...
Can you imagine the guilt Lan Zhan could have felt? He snapped at Wei Wuxian in the cave, and yeah they worked together to kill the turtle, and Lan Zhan sang to him. But you and I both know he was ruminating over calling Wei Wuxian a loathsome person because he didn't mean it; he was just hurting, and maybe in some hidden recess of his heart Lan Zhan knew Wei Wuxian wouldn't judge him for being vulnerable.
And then the next time he sees the boy he can't not be endlessly curious about, the boy has changed irreparably. Lan Zhan knows it can't be because of him... but... but what if he'd been nicer? What if Wei Wuxian only sees him as a cold, crying, ungrateful person?
What if the boy he trusted enough to cry in front of never comes back? What if it's somehow his fault?
The Yiling Patriarch is a living legend—a terrifying, ancient force of nature, dispensing punishment or reward with implacable, indifferent fairness.
Wei Wuxian, on the other hand, is a weird but oddly charming guy who wanders around the cultivation world making fun of people's art and mooching food from sect leaders.
It really upsets people to find out that they're the same person.