There is no particular reason mentioned for how the disorder started. Repeatedly you see things shattering and an accident which is supposedly the cause of it that too by assuming. The bird becoming many birds and cloning? splitting? somehow he got the disorder.

No mention of parents or family in the story. Only one friend who shows up. Really do not understand how he makes enough money to eat and live. And spend on hobbies of other personalities.

Somehow the doctor lies to him or manipulates his reports and somehow he is fine with it. He was being experimented or something. MK Ultra type I guess. He is fine with the other personalities and he just hates hearing the same song while putting the trash out or seeing the same shows on tv. I mean he can watch any other show or listen to any other music. It simply doesn't make sense.

Two personalities like the same lady but she likes one. Can atleast say to her that he has a disorder, but no, he hides it from her. I mean you cannot trust the person you love?

Who writes notes when they use the same smartphone. And it seems like he has bi polar and it has two personalities which split further or something. One side is way too soft and other side is way too rebellious. And this is just my assumption.

Evil side tries to kill the good side and the many splits merge together. You just lose interest and patience at some point.

But do not watch this if you want your time back and do not take anything which is happening in the movie seriously. Think if it as a thought experiment or something. If anything does not make sense do not try to find answers in the movie. It is just a waste of time.