Spoilers ahead maybe??> I absolutely adore this movie It of course scared me but also made me sad, I felt more grief stricken after seeing a theory on it, in which I will summarize somewhat... (I am leaving a bunch of things out) In the beginning there's the scene of Kevin falling down the stairs but the whole went into a coma after didn't exactly make sense (I've got a personal experience where my little brother fell down a flight of marble stairs into the basement which results in a small bruise and ice pack, and he did not go unconscious, but it does vary from person to person) so the theorist suggested Kevin had a tumor and falling down a flight of stairs and hitting his head caused him to fall into a coma after (assuming) he was taken to a hospital they found the tumor and proceeded to do surgery on Kevin and (assuming) the tumor was in his Frontal Lobe the easiest way to remove the tumor is via the eye, hence the "Put the knife in your eye" Kevin could feel the surgical knife and other instruments in his eye, which would hurt. And at the end with the "572 Days" near the end along with the voice beckoning Kevin to sleep, which means he probably died at the end of the 572 days in the hospital.
If you want the full thing cause' there is a lot more info on this point of view of things like what happened to the parents, what's the monster, and where did Kevins sister go... So go check out MatPat's Film Theory on it on YouTube or prime video where I saw it.