I guess it is somehow based of the real fear of "what if I cannot differenciate between dreams and reality?".
It already happens that some dreams can feel so damn real while you are having them, so when you wake up it takes some second to realize you have woke up and also to leave that lingering feeling aside.
For me, the story stays alive because of this twisted core of reality Vs dreams. The FL doesn't tell me anything since she is your classical victim and the others are there for show but the male lead has some nice depth within him.

This is one of those stories where I actually don't like it that much for some reason, but I LOVE a loooot of things it does do.
Someone else summed up how realistic and moving this type of regression story is as the regression actually breaks the ml down mentally, but within that I want to highlight the paranoia and constant questioning of reality the ml is going through.
That's so crazy and yet so true to think about if he really slides down the rabbit hole of: Is anything even real? What if I wake up and it was just a dream? A dream in a dream? Do I wanna know?
Afterall, who says any of this has to be real?
Lastly, tie in the other characters blind devotion to him and the fl too, and the scale of impropability truly does tip.
Fascinating.