I just realized something: LWJ might blame himself for WWX's death – a long thread.

You all might go “Well, no shit” but stay with me.

The way lwj treated wwx before his death and the way he treated him after his resurrection are different. This is well noted in the novel.
Since he met him and till the day he died lwj loved wwx but couldn’t express his own feelings.

Perhaps he thought that loving such a person is wrong or perhaps he couldn’t really express his feelings in behavior and words, that’s why he got angry, or didn’t react, or said mean things. I think LWJ hates that part in himself because

he does get jealous when he sees wwx with his friends, while he himself is alone. He hates that part in himself and that’s why the only way he is able to express his feelings is through biting, kissing without the other knowing, and most importantly – composing a song.

Wwx, and the people around him (JC, NHS) are sure that LWJ hates his guts. Wwx, being a friendly person himself, cannot understand that this behavior actually comes from deep feelings. He thinks that lwj truly hates him. At first he still tries to get along

For example:

“We are so familiar and you’re still like this”
“We saved eachother and you’re still acting like this”
“Aren’t we already friends?”

It then goes to wwx accepting the fact that lwj hates him, although he can’t understand why:

“Even if people don’t like my character at first they still come to like me later” (as opposed to lwj, conversation in the Xuanwu cave).

And last, after he turns into the yllz, wwx accepts lwj’s dislike of him to the fullest.

Example:

“After all, on the topic of how my heart is, what could other people know about it? Why should other people care about it?”

Lan WangJi paused. He had suddenly been angered, “… Wei WuXian!”

WWX says it to lwj, which hurts lwj cause he loves him. But he still can’t say it. So, shocked by such a claim, he shouts his courtesy name, the way he never did before.

And last: Wwx throws this truth at him at their final battle at nightless city:

Wei WuXian spun around to dodge the attack and laughed, “Fine, fine. I knew since the start that we’d have to fight a real fight like this one sooner or later. You’ve always found me disagreeable no matter what. Come on!”

Hearing this, Lan WangJi’s movements paused, “Wei Ying!”

LWj slowly comes to realize that he wasn’t there for him. That his behavior finally let wwx understand that lwj hates him, never cared for him, and now, that he is completely alone, when lwj is the only one still willing to stand beside him, wwx thinks that he wishes to fight him

That’s what brings lwj to realize that he might lose him, the only man he has ever loved, and the man whom he should have treated differently. I think LWJ is truly afraid at that moment. And that’s what brings him to do the hardest thing he could ever do– confess his feelings.

After finally taking him out of nightless city he brings him to the cave and spills everything that he held in his heart since he was 15. However, it’s too late. WWX is not listening to him, he is too devastated, too weak, too alone, and he doesn’t believe in anything and anyone.

If lwj told him those words in an earlier time, perhaps wwx would have reacted differently. But the only thing he could say is “Get lost.”

Lwj realized that it was too late.

After all the years of wwx being the one who chased after him and paid attention to him, and him being unable to react properly, now when it’s the other way around, wwx gives him the final and most cold answer – now it’s lwj’s turn to be turned down.

Understanding this, the only thing lwj can do for him at that moment is stand by his side. He shelters him and harms his own clan members for his sake. He takes the most severe punishment for his sake. However he was too late.

Too late to let wwx know he is not alone and that he is loved, truly loved. Then WWX died.

He died with no one by his side, without anybody to care for him.

LWJ was in seclusion that time. Of course he wasn’t the one to directly cause his death, but I have this feeling that the regret literally ate him at that time, and in the next years to come.

Every day without wwx was a day he regretted he wasn’t be better, wasn’t able talk to him, listen to him, be friendlier to him, stand by him.

That’s why the only thing he can do to atone his sin is to cling to the things that wwx left behind (a-Yuan, looking for his sword and flute) or the things that were so wwx in nature, the things that he scolded him for (keeping rabbits in Gusu, bringing Emperor’s Smile).

Its’ as if he wishes to bring wwx to life by taking all his things and accepting them the way he never did, to show that he cares, he always cared.

In the heat of the moment, he brands himself with the Wen’s iron, to share wwx’s pain and to carry his burden with him (just like he told lxc and lqr during his punishment).

Then 13 years pass.

Who can imagine the way hgj’s serious face looked like the moment he heard the flute on Dafan Mountain? How big were his eyes and how fast was his step when he heard people scream “Ghost General?”

How fast did his heart beat and for how many seconds did he stop breathing when he heard his song played for the first time by another?

He was given another chance, and he will not miss it. He will do things differently this time.

“Mark your words” and “I’m taking this man back to Gusu with me” is how he began his redemption. And in the following months, he treats wwx the way he never did: more caring, more talking, even joking and retorting back. In short – more communicating.

Thinking all this time that wwx actually remembered his confession in that cave, he accepts all of his teasing, he accepts all the hurt, knowing that he deserves them. He is still by wwx’s side, stands by him, carries him, without even asking. He just does it.

He is a different person, and even wwx notices that. He’s been given a second chance and he will not lose it. He will not let the love of his life slip between his fingers again.

That's it! Thanks for reading this far

And to end it on a good note:

Wei WuXian held the ribbon in between his lips, using both of his hands to retie the hair that had come loose underwater, “Be honest about whether or not you thought about me in the same way.”

In a solemn tone, he spoke, “Rejecting me like that so coldly every single time—it really made me lose face, don’t you know?”

Lan WangJi, “You can try, now, to see if I would reject you over anything.”

That last sentence...I think it's the best conclusion to this whole thread