The rumor says that Totoro is the God of Death, so the persons that can see Totoro are actually close to death, or already dead.
There’s a famous murder case called The Sayama Incident (My Neighbor Totoro takes place in Sayama Hills) in which two sisters turned up dead.
“Everyone, do not worry,” wrote Studio Ghibli. “There’s absolutely no truth or configuration that Totoro is the God of Death or that Mei is dead in My Neighbor Totoro.”
The Totoro Death God theory isn’t new—but it’s a theory that will not die.