It was mostly just due to MC misunderstanding and acting out against his his good intentioned brother, and said brother eventually giving up on repairing the relationship? Felt a bit anticlimactic for them to just handwave their bad relationship as brothers away with a "it was just a misunderstanding".

MC had a bad childhood, which later makes him incapable of love. His family favored the ML as his elder brother, blamed him for his mother's death, and would pit him against the ML. For example, no one in the family had ever even celebrated his birthday because it was also his mother's death day. His father didn't treat him badly but he also gave him no family affection when he was young, and wants to compensate him later on because he feels bad. After he finds out they are not blood-related, ML starts to feel attracted to the MC. The MC grew up obsessed with becoming or beating the ML and pushes the boat along.

It just really feels like it's not worth it for the MC. ML is very good to him later on but it feels too little too late. MC wasn't perfect, he stumbled the ML a lot of times, but he always had a bottom line, e.g. He found proof that the ML was gay but decided not to use it. Meanwhile, the ML was the one who revealed MC wasn't the biological son - even if he didn't do it with malicious intentions, he still is the one who overturned MC's life. The ML tells the children in the family not to call MC names, but no one in the family gets any retribution for how they treated the MC other than a "don't call him that" scolding. And I can't help but feel that the ML basically was the root of why MC grew up with a distorted mentality, even if it was not his fault. As MC says, meeting the ML was a "fated tragedy." MC is really mature though, he doesn't feel like he needs an apology or any compensation for how he was treated.

Particularly talking about how Zhiyuan immediately changed his surname, resigned from Cen An and tried to return the Cen's assets given to him

aka when zhiyuan basically asked a rhetorical question to zhisen if zhisen thought ning zhe could be a good little brother to zhisen after being alienated and instigated by the family for more than two decades since they were a kid

the deeper meaning is that MC really doesn't care about owning this or that from the Cen family, all throughout his life he just wants Cen Zhisen. Whether as his brother previously, or later on as his lover

one of my faves extra where a few days? I think before the paternity test was revealed.... Cen Shengli (their dad) was just diagnosed with brain tumor (?) iirc and went through surgery. Both Zhiyuan and Zhisen were the only one waiting, and Zhisen, knowing that Zhiyuan barely ate, told him to eat something. Zhisen also reassured Zhiyuan that everything would be fine, seemingly knowing Zhiyuan's restless thoughts about their dad's surgery.

He borrowed an umbrella when it was raining hard as they exited the hospital just so Zhiyuan wouldn't drench himself in the rain like he was about to do, suggested to Zhiyuan to compete fairly with him (about the CEO position) when Zhiyuan started being sarcastic because of Zhisen's (seemingly unprovoked in zhiyuan's eyes) caring attitude.... walked Zhiyuan to his car first and stood silently there with the umbrella, until Zhiyuan's car completely disappeared from his sight.

This is shown even in the main texts like how Zhisen silently stopped the reporters from bothering Zhiyuan when their matters of Zhiyuan being not blood related had just become public, or how the "guest room" in Zhisen's apartment was initially for Zhiyuan, because he wanted the both of them to live together as soon as Zhiyuan came back from the US but he lost the chance to do so.

Or how, long before this, one day during New Year's eve as Zhiyuan worked overtime, Zhisen who initially already had scheduled plan with his "little boyfriend" and only came back to the office to fetch something, once he saw the lights in Zhiyuan's office was still on.... he instead, sat down on the sofa, silently accompanying Zhiyuan through the night even without Zhiyuan knowing, ditching his scheduled plan.

Zhisen only explicitly said that he investigated this matter truly not because he wanted to make Zhiyuan lose to him

For me, the hints were how Zhisen accidentally met Ning Zhe through a project. His face was so similar to his biological mother, like who wouldn't get suspicious?

and later on Zhisen did reveal that on that day, he saw the red mole on Ning Zhe that was "gone" in Zhiyuan. Zhisen specifically said that he was there the day Zhiyuan was born and noted the red mole on his arm (I think it was on the arm?), but then the next day that red mole was gone. The adults that he told it about either said Zhisen was delusional (lmao) or the mole might be gone after birth.

Do you get what I get here?

Do keep in mind that Zhisen cared about Zhiyuan, so, so much even way back when they were still only brothers.... to the point that his classmate (more like Zhisen's first plaything) said to Zhiyuan in a jealousy, that, unfortunately Zhiyuan was only Zhisen's brother.

Do you get why Zhisen did the paternity test now? (No it's not because Zhisen have crooked thoughts abiut Zhiyuan when they were still brothers)

the Main Couple didn't have a really close brotherly relationship, mostly because they rejected each other, one felt constantly abandoned, surrounded by whispers of rejection and the other was too proud to give all his attention to someone who keeps pushing him away, so they gradually drifted apart, keeping only a conflicting husk.

But their minds weren't too different: one was obsessed with the other, hating his own inability to stop searching for the presence of them and trying to distract his loneliness with frivolousness, not unaware of it though; the other seemingly indifferent and with little truly on his mind, alienating his feeling but in the most hidden corner he couldn't get rid of the person, and from time to time acting out of his way to keep in touch.

MC finds out his older (ML) is not his biological brother, something the ML investigated after much suspicion and it got revealed one way or another. After that whole scenario, MC just went along with the whole thing and there was very little reaction from him... pffttt. Then, the ML started showing him care and attention, like dude, you have been distant for years. Now, they are almost always together and all that.

like ZY changing his surname from Cen to Ning immediately he found out about his identity. Another thing is that the reveal is prolly a catalyst for them to be able to get together and all that (maybe for the 'we ain't real brothers' effect).

real brother of the ML (Ning Zhe or should I say Cen Zhe). As at chapter 31, he is just cool and not doing too much like revenge for time lost with his real family, fight for asset, and all that. Dude just continued with his job and stays with his Ning fam. Maybe cos they are good parents but there are so many novels where he would begin to show greed especially since his real fam is wealthy.