While re-reading volume 1 of the manga, I thought that the reason for Sarina's reincarnation might be the disease (anaplastic astrocytoma) that she suffered from.
Reason for this: If you take into account the meaning of each kanji character in anaplastic astrocytoma, it becomes a literal translation, which means that it is a cell tumor that loses its shape and takes on the nature of being able to house stars. In other words, it can be thought of as a disease in which you die once and then become a person who houses stars. However, this is merely the reason why Sarina was able to reincarnate. Goro-sensei was able to reincarnate, although he does not exist in Sarina's memory, but before her reincarnation, she negotiated with God to let Goro-sensei be reincarnated as well, and as a result, he was given the conditions for reincarnation, or rather, his destiny after reincarnation.

The reason why Sarina-chan and Goro-sensei were able to reincarnate | AKMT!!'s post