Wei WuXian's feelings regarding Jiang Cheng in his new life
Let's take a look at Wei WuXian's attitude and feelings towards Jiang Cheng in his new life. Rebirth/reincarnation is used to represent a restart where past life's debts and gratitude are no longer lingering or connected to Wei WuXian anymore.
First, when Wei WuXian meets Jiang Cheng again:
Hearing the voice, all of the blood from Wei WuXian’s body traveled to his head and drained away a moment later. The good thing was that his face was already a pile of white. A shade whiter wouldn’t make too much of a difference. — Chapter 7
After the shock, Wei WuXian just feels numb and, later, he thinks about Jin Ling only. Then, when Jiang Cheng suspects of him and attempts to bring him back to Lotus Pier (to torture him), Wei WuXian tries to escape and succeeds with Lan WangJi's help.
Next, when they meet in Qinghe:
Jiang Cheng pulled a curt smile on his face, “… Don’t you have anything to say to me?”
With a sincere tone, Wei WuXian replied: “I don’t know what to say to you.” — Chapter 24
Wei WuXian had nothing to say to Jiang Cheng. But when Jiang Cheng spoke malicious things about Lan WangJi, he got annoyed. And when Jiang Cheng brought up Jiang Yanli and Jin ZiXuan, he felt bad. When Jiang Cheng talked about him going back to Lotus Pier to ask forgiveness, he calmed himself down though; Wei WuXian didn't want to go back there.
Wei WuXian calmed himself down and searched as fast as he could for a way out of the situation at hand. Although he had always dreamed of returning to Lotus Pier once more, he didn’t want to go back to the tattered one nowadays! — Chapter 24
Then, Wei WuXian's actions when Jiang Cheng meets him and Lan WangJi in the Jiangs' ancestral hall:
Doing this, Wei WuXian wanted to keep it away from Jiang Cheng in the first place. Now that he discovered them, he knew they’d definitely be faced with some vicious remarks. He didn’t want to argue, “I didn’t take HanGuang-Jun to the other, more confidential places of Lotus Pier. We only came to greet Uncle Jiang and Madam Yu with a few sticks of incense. We’re finished already and we’ll be leaving now.”
Jiang Cheng, “If you’re leaving, please go as far as possible. Don’t let me see or hear you fooling around in Lotus Pier again.”
Wei WuXian felt his brows twitch. He saw Lan WangJi’s right hand press onto his sword and stopped him at once.
Lan WangJi turned to Jiang Cheng, “Watch your words.”
Jiang Cheng was quite blunt, “I think you’re the ones who should watch your actions.”
Wei WuXian’s brows throbbed even harder, and the ominous feeling within him grew as well. He spoke to Lan WangJi, “HanGuang-Jun, let’s go.” — Chapter 87
Wei WuXian wasn't willing to deal with Jiang Cheng at all; he was quite impatient. When Jiang Cheng said even more nasty things about Lan WangJi, he couldn't bear it and started to fight him. He regretted it pretty soon though; to fight with Jiang FengMian and Madam Yu's son in front of them.
Then, Wei WuXian's decision:
“Lan Zhan… Let’s go.”
Go. Right now.
Don’t ever come back again. — Wei WuXian, chapter 87
While Jiang Cheng vented about the golden core in GuanYin temple, Wei WuXian was pretty quiet. When Jiang Cheng was done, that's Wei WuXian's attitude:
Wei WuXian hesitated, “… You don’t need to say sorry.” (...) “Take it as my repayment to the Jiang Sect.” (...) Wei WuXian pressed his temples, “Forget it. It’s all in the past now. Let’s not mention it again.” (...) If this were exposed in the past, he’d most likely laugh and comfort Jiang Cheng (...) But now, he indeed didn’t have the strength left to put up such a confident, nonchalant pretense. — Chapter 101
Wei WuXian was tired.
Lastly, before leaving GuanYin temple with Lan WangJi:
Wei WuXian took the flute. Remembering that Jiang Cheng was the one who brought it, he turned over there and commented casually, “Thanks.” He waved Chenqing, “I’ll… be keeping this?”
Jiang Cheng glanced at him, “It was yours in the first place.”
After a moment of hesitation, his lips moved slightly, as though he wanted to say something else. However, Wei WuXian had already turned to Lan WangJi. — Chapter 110
In summary, Wei WuXian is pretty short with Jiang Cheng after being reborn; he doesn't want to deal with or to be involved with him at all. There is no positive sentiment left in him for Jiang Cheng. He's rather indifferent too.
One popular interpretation is that the reason behind this is that Wei WuXian feels guilty. That's not really in character though.
If it was because Wei WuXian feels guilty and remembers things he doesn't want to remember when he sees Jiang Cheng, why he doesn't feel the same way about Jin Ling? Wei WuXian feels really guilty about Jin Ling; Jin Ling grew up without his parents because of him.
No matter what prey haunted Dafan Mountain, he couldn’t take it. Wei WuXian could fight for it against anyone, except for Jin Ling. — Chapter 8
But Wei WuXian didn't avoid Jin Ling; he even took over the responsibility of teaching him.
Wei WuXian didn’t expect Jin Ling to appear here at all, much less how arrogant he behaved, thinking to himself, I wonder how did this child’s personality turn it this way, with a poor temper and strong hostility. He learned all of his uncle and father’s faults, but none of his mother’s strengths. If I don’t do some tinkering with him, he’d definitely suffer great losses in the future. — Chapter 20
Similarly, Wei WuXian feels guilty about the people he killed in the Nightless City massacre; he'll remember it for the rest of his life but this won't make him stop living.
"The things I did, not only do you remember them, I remember them too. You won’t forget them, and they’ll stay even longer in my mind!” — Wei WuXian, Chapter 82
Wei WuXian also felt guilty when he found out Lan WangJi had been in love with him the whole time while he tried to disgust him, essentially playing with his feelings; but he only focused on confessing and clarifying everything.
He suddenly felt terrified. If Lan WangJi didn’t know that he couldn’t at all recall what happened in the few days after the massacre at Nightless City, if Lan WangJi thought that he knew about his feelings all along, just how horrible were the things he did after he came back?
(...)
Wei WuXian really couldn’t keep on thinking any longer. He rushed towards the Guanyin Temple’s doors, and cultivators immediately stopped him. — Chapter 99
Wei WuXian's heart is really strong and he's always moving forward, he's not a stereotypical character.
So, Wei WuXian doesn't treat Jiang Cheng the way he does in the present because of guilt. Of course, it must be because of the first siege. Jiang Cheng lead it, that's no rumor. One of the several mentions about this:
During the siege in Luanzang Hill, aside from Jiang Cheng, Jin GuangShan was the second-greatest contributor. — Wei WuXian, chapter 7
Wei WuXian died from a backlash, but it wouldn't have happened if he wasn't pushed too far. The only other times he lost control were: in QiQiong path, and when Jiang YanLi was killed right before his eyes.
A clear example of how Wei WuXian feels about first siege when children were playing Sunshot Campaign:
“Jiang Cheng”, “Hmph, I can’t be better than you? Do you remember how you died?”
The light smile on Wei WuXian’s face dissipated at once.
It was as if he had suddenly been pricked by a poisonous needle. A faint, sharp pain came from all around his body. — Chapter 32
Remembering the siege, Wei WuXian feels pain run through his entire body. And not only he died there; the Wen remnants died too, and they were like family for Wei WuXian.
He was thinking, To walk it (single-plank bridge) all night… huh?
It wasn’t really that dark at all. — Wei WuXian, eating with the Wen remnants, chapter 75
He turned around, knowing that it’d be a long time before he’d get to see the people he was familiar with again.
But… right now, wasn’t he on his way to seeing people he was familiar with as well? — Wei WuXian, chapter 75
The tall and the short, the men and the women, the old and the young—all of them were demons drenched in blood. But on these figures, Wei WuXian saw a few familiar shadows.
(...)
Wei WuXian’s lips trembled. He seemed as if he wanted to say something, but still he couldn’t say it. He lowered his head and bestowed upon them a heavy salute. His voice was rasp, “… Thank you.” — Chapter 82
I personally feel, Wei WuXian is not one to resent people. But he can't pretend nothing happened either.
Also, Wei WuXian knows from a long time ago that his and Jiang Cheng's cores, their three views, are too different; they're pretty incompatible. Wei WuXian no longer has to put up with Jiang Cheng, since he already fulfilled the promises he made.
And just as Wei WuXian said, “I don’t know what to say to you” in chapter 24, by the end of the story Jiang Cheng is finally on the same page as him, and also thinks there is nothing left to say between them.
“There’s nothing to say.” — Jiang Cheng, chapter 110
