This is just my opinion. From what I understood of AnRan being reborn is that after ML knows about the truth, he made his life worst than ever. After all, AnRan's used the fact that Sui Sui's love for Mr. Fu as a fan. Feeling betrayed, Mr. Fu alienated on AnRan and so on. When AnRan had no choice but to marry another wealthy person, his life became worst. I got a feeling that when Sui Sui returned, it was also the time AnRan returned, as well. The moment I read a certain part of the novel at the beginning. I suspected he returned and I was right when I read the part he said he was reborn.

Instead of this couple coming out clean about what happened, MC finds out by accident.

The fake mother didn't want to tell MC because they stole babies and MC is from a strong family. By this point the fake father is sick and dying.

AnRan spread the info about their circumstances (baby switched) and also found a way to spread the rumors about MC bullying him. He also promised to look for an organ donor for the fake father, but never really did. So, to finish MC, after ignoring the couple for a while, AnRan went to visit them and act pitiful and filial. Instead, the fake mother confronted him about he ignoring them, not visiting them and spreading fake rumors about MC (because at this point MC is the one taking care of the couple and with a bad reputation he can't make money for them). AnRan gets mad and shows his real face, confessing MC never bullied him, but also accusing fake mother of lying about MC being her biological son. MC heard it all and the whole thing was streamed by someone else.

The couple had a baby, but the baby died. Fake father switched the dead baby with MC and the mother switched MC and AnRan.

Dead baby went to MCs biological family, MC went to Jian and AnRan went to the couple.

they switched two times because fake mother and fake father acted separately.

I have no idea why ML fell in love. I always felt that the main reason is because MC knows a lot about ML, things from when ML was not famous yet and apparently almost no one else knows despite ML being super famous now. And then other things like MC being too silly, too s*upid, etc. A rare pure person in the middle of the dirty, it seems.

MC fell in love pretty fast. At first he decided to be away from ML because ML originally was meant to be with AnRan and anything related to him may leads MC to his death. He said that, yet, he never really avoided ML or tried to stay away. ML treated him so specially, it's so obvious, yet, MC never realized - the author really exaggerated MCs lack of awareness.

Funny (or not so much) thing, even after ML acted all different with MC, he still said he had no intention to marry and that no one attracted him (after the idol competition). He shows interest, then deny, then ask someone else's help with MC, get jealous and then refuse a relationship. He sometimes seems to know he likes MC, he sure act like he does, but when push comes to shove, he steps back just to go back in track right after.

Right now MC is kind of annoying me, he is even crying because ML burned his finger and cut pineapples later without telling him. He didn't even cried when people were telling him to die, he had no money and fake father dying. And while the author may say that he is like this because he doesn't care about himself and only about others, honestly, I don't buy it. To me, when MC cried about something ML was enduring and fine, MC made this about him (just like when AnRan acted pitiful) because people will naturally console the crying guy and not the one who is truly hurt. I mean, why are you crying when the real hurt person is enduring? Sorry if it sounds too harsh, but that's just my opinion. A nun once told me that we shouldn't sit in the "victim's little chair" when you're not a victim or shouldn't / doesn't want to be tagged as such.

EDIT: and then, when their relationship is finally clear, MC convinced ML to hide their relationship and he said he is not ready to tell his family just yet (while some of them are more than ready to hear about this). I mean, MC is really good shooting his own feet.

that's when it gets revealed to everyone, and I mean like the whole world, that MC was swapped at birth and was the actual victim of Anran's bullying and that Anran is a green tea lotus bit*h - it's super satisfying to read the confrontation and truth being revealed in the best (for MC) /worst (for Anran) way