It's not entirely confirmed, but it seems likely that Sekai hates Maya for one major reason:
- Maya broke Sensei beyond repair, after the Accident.
Noriko reveals a lot about Maya, but one of the major things is how she believes Maya fucked Sensei up for good:
As far as has been revealed, Maya was the one who pursued the relationship with Sensei:
We also had Pareidolia seemingly disliking Maya for how she groomed Sensei:
And we know that Maya tried to keep those that loved Sensei away from him, apparently convincing him he was all alone:
We also had Sensei mentioning he was willing to be Maya's toy last update:
And, Ami has brought up that she suspects something must have hurt Sensei to cause him to stop tutoring:
It seems like the only thing that could have hurt him, and the main thing that sends his body into shock, is him remembering his relationship with Maya.
As fucked up as it sounds, it seems like a Wizard took advantage of a mentally ill man in his 20s, and broke him into his modern self in his early 30s.
Overall: Sekai probably dislikes Maya for what Maya did to Sensei mentally.
Not to mention, if Sekai is just some kind of hallucination from Sensei's mind, it might be the part of Sensei that regrets and dislikes what he did with Maya, personified. Just, you know, blaming the Wizard.