Yume Nichiroku (夢日録)/theories

Reason for Hirosae's Isolation is ERIC.
In City accydent Event clearly see that victim of accident its Eric from UBOK. It seems Hirosae was or maybe even still is good friend of Eric. In UBOK endings Eric was hit by car, Hirosae was witness of this accident and after she understand its Eric its make her very sad, she thought that Eric died after that and isolate herself from the world. Possibly, she blame herself for this, but fortunately in secret and true ending of UBOK Eric is survived and wakes up in hospital room, alive and well, possibly note that he found in his hospital room from Hirosae.